Well, actually there is only one. The just released SONAR 5 is the the first and only native 64-bit digital audio workstation on the market. SONAR 5 ships as both a 32-bit and 64-bit application, and the 64-bit double precision floating point engine is accessible in both applications. Cubase is not a 64-bit coded application. There is a big difference.

What is the real world difference between a program written native 64 like Sonar 5 and a program written to be compatable with 64bit CPU's like Cubase 3 SX?

"In addition, Cubase SX3 supports a 64-bit operating system and up to 4 GB of RAM when used with Microsoft Windows XP 64-bit Edition and 64-bit processors such as the AMD Opteron. According to Steinberg, the doubling of maximum available RAM resources yields a significant performance boost when used with the latest generation of 64-bit processors."

Is it just marketing?

This x64 architecture includes 2 new enhancements that translate into direct benefits for music and audio software uses: the ability to access more physical memory (RAM), and more internal CPU registers. In the case, of SONAR 5 x64, as a 64bit coded app, it can access and use up to 1024GB, of RAM, for instance. Whereas, SX3, because it is not a true native 64bit coded app, can only access 4GB. According to Cakewalk, there is a 20%-30% performance gain from 64bit computing, over 32bit. You may want to read the following Cakewalk white paper on 64bit computing and it's audio implications:

local music shop

Or this article interview, with Ron Kuper (Cakewalk CTO) from several months ago, concerning the benefits 64-bit will bring to SONAR 5:

http://www.digitalproducer.com/articles/viewarticle.jsp?id=30309

I just got my SONAR 5 upgrade, but unfortunately 64-bit computing will have to wait, until I upgrade my DAW from 32-bit. But since SONAR 5 includes both 32-bit & 64-bit versions of SONAR, I will be ready to jump in once I do make the upgrade and I won't have to pay extra for it either or use a stupid dongle.