we want to migrate our database to exadata.which OS is better for Exadata?Linux or Solaris?

One point is dba's is supported on Linux but not on Solaris because of the absence of fuse support.

Apart from that I believe some of it depends on your familiarity with one OS over the other. Many of the advantages of Solaris on Sparc are due to the sparc architecture and you will not gain those here for example hugepage size is actually limited by the intel architecture on Exadata.

Many admins swear by dtrace. hat may be one reason to choose Solaris over Linux.
Then again Linux admins may be cheaper than Solaris admins.

Ref metalink note : 1054431.1

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Solaris
Let's start with Solaris. Solaris most definitely is Unix while Linux is not. To be clear — Linux is a kernel, Unix is an operating system. The Unix operating system contains a kernel which is usually supplied and supported by one of the Unix hardware vendors: IBM, Sun or HP. On the other hand, Linux distributors incorporate the Linux kernel – along with other utilities, which forms the crux of the distribution.
Support: Solaris has support from Sun. If handholding and support is important to you, you won't really find it with Linux – because no one really owns Linux. If you're a SPARC guy that is not really interested in running your OS on x86 systems– than your decision becomes that much easier. Solaris is optimized for the hardware -- a big plus. If you're a true believer in openness and like a certain separation of church and state, than this becomes less of a perk. Sun provides four different types of support programs for commercial Solaris: platinum, gold, silver and bronze. It should be noted that unlike other Unix variants such as IBM's AIX, that Solaris also runs on the x86 architecture.
Maturity: Unix has been around much longer than Linux and unquestionably is a more mature operating system than Linux. Solaris is one of the big-three Unix variants (AIX and HP-UX being the other two), and has been around much longer than Linux. While Linux has made great strides in recent years, even the most avid Linux proponents understand this issue. It's just an easier sale to run mission critical databases on Solaris than Linux. Some corporate CIO's still view Linux as a hacker operating system, and will not let you run their financials on Linux. While I don't necessary agree with this, sometimes we need to follow the money, especially in these challenging times.

Linux

Platform independence: While Solaris is available on many different architectures, including x86 and their RISC Architecture, Linux wins in the "platform independence" category. You still cannot run Solaris on an IBM Mainframe (this is relevant because Linux actually has a growing user base on the IBM System z) nor can you run it on most HP Systems or any IBM midrange systems for that matter. The bottom-line is that no other operating system provides the hardware choices that Linux does. In a corporate environment with different types of architectures, you can consolidate operating systems around Linux and save a boatload of money. Because of this, OS consolidation is actually one of the hottest projects that many IT organizations are currently involved in.

Support: While Linux does not claim to have the hardware support that traditional Unix vendors provide, in recent years the top Linux distributions have really improved their levels of OS support. For example, Red Hat offers three support options, Premium, Standard and Basic. Premium comes with 24x7 and one hour support. Other companies are adding to this. Oracle is an example of a mainstream company that has come out with their own distribution: Oracle Enterprise Linux, along with the support package, Oracle unbreakable Linux, competes directly with Red Hat.

OpenSolaris

What about OpenSolaris? OpenSolaris is based on Solaris. While conceptually discussed for years, the project was really kicked into high gear in March of 2007, when Sun announced that it had hired Ian Murdock (Debian founder) to lead what was than referred to as "Project Indiana." The end result of this project was to complete an OpenSolaris distribution. It is similar to Linux in many ways – as vendors acting as distributors (similar to Red Hat and SUSE) could build on the OpenSolaris kernel and sell the distributions as their own. OpenSolaris is actually a set of source code encompassing many things outside of the actual kernel, including; libraries, commands and X. Distributors are than free to pick and choose from whatever source code areas as they see fit -- they don't have to take all of the OpenSolaris source. Some examples of distributions are Nexenta, BeleniX and Schillix. It's worth noting that Sun has announced that future versions of its commercial Solaris operating system will actually be based on technology from the OpenSolaris project.

Even OpenSolaris comes with a variety of support options; including phone support, on-line support and automatic notification of package updates and fixes directly from Sun. With OpenSolaris, users have two options: production support and essential subscription support. Production support is 24x7 and promises a one hour response time for priority calls. OpenSolaris essentials is $324 per year with on-line support only delivered within 48 hours. What's new? On December 1, 2008, OpenSolaris 2008.11 was released, in versions that could be booted as a direct install or using LiveCD. Among other features, this release also includes ZFS, Sun's newest filesystem.

Linux distributions will use the Linux kernel and OpenSolaris distributions will use the Solaris kernel.

So what's the difference between OpenSolaris and Linux? First and foremost – Linux distributions will use the Linux kernel and OpenSolaris distributions will use the Solaris kernel. This important to note, because while Red Hat Enterprise Linux is built entirely from open source Linux., Solaris is (or was until recently) built entirely from closed/propriety engineering efforts by Sun Microsystems engineers.
While I applaud Sun for getting on the bandwagon, and while it is true that imitation is the best form of flattery, I'm not sure if Sun might be a little too late to the dance. Linux just has too much momentum today, and trying to re invent Unix this way seems like a long-shot. Linux today is a real enterprise solution, used by almost all of corporate America, and the only OS that is actually growing in sales. On the one hand, while it is the hope of open system aficionados everywhere that this Sun venture succeeds, I'm not certain that as a company, Sun would have been smarter to stay with pure Solaris and their SPARC-based architecture and played to their strengths. This is what IBM has done around their System p architecture and it has served them very well in recent years. At the end of the day, you are what you are. I'm not certain that Sun really knows where they are today.
About the author:Ken Milberg is a systems consultant with two decades of experience working with Unix and Linux systems. He is a SearchEnterpriseLinux.com Ask the Experts advisor and columnist. 

Thanks
Srini