IMHO the only opinion to use the 64bit kernel is if you have more than 32GB of RAM installed, performance wise it doesn't make a difference (or maybe I just used the wrong benchmarks or software that doesn't stack up with the hardware).
You'd want to test whether your 64-bit apps ran better on the 32-bit kernel versus the 64-bit one to see any noticeable differences. There is some small overhead in adapting between the two modes when there are lots of round trips between app and OS.
However, many "real world" benchmarks folks use have major disk I/O bound segments in them. That will swamp any small incremental benifits to matching kernel and app on bit width.
There can also be an effect before the level of 32GB of RAM is installed:
" ....Regardless of page table organization, this table eventually grows until it contains (at minimum) an entry for every page of physical RAM in the system.
....
For a computer with 64 GB of RAM, given a 4 KB page size, the OS must manage almost 17 million pages of physical RAM, each of which has a page table entry and a vm_page structure. In total, these data structures would potentially consume well over a gigabyte of kernel memory by themselves. In a 32-bit (4GB) address space, this would significantly limit the kernel address space available for other purposes. ... "
http://developer.apple.com/mac/libr...html#//apple_ref/doc/uid/TP40001064-CH227-SW4
If very often use all of the memory installed as you approach 32GB things are getting worse. The example above is for 64GB of RAM using 1GB of page mapping space. It consumes memory to track the memory you are using (which uses more memory). Even at 32GB of RAM that's 500MB of space just to a single data structure. If you app(s) are ripping through loading lots of random disk blocks that need to be cached, that is a competing usage.
It is getting easier for folks to get up in the 24+GB range with the Mac Pro now. Used to be the 4MB modules were "expensive". Now it is 8MB ones. In a year or so the 4's will be affordable by much more of the mainstream and the 8's will be more tolerably priced for those with high need.
For folks who over provision on RAM ( e.g., install 16GB but only rarely rise up over 9GB ) then yeah... 64-bit kernel isn't going to get much of speed boost. However, it also isn't much of a space hit to use 64-bit kernel either because have all this extra buffer just lying around anyway.
Note that if you only have sub 4GB of memory somewhat don't want to use 64-bit kernel because it has a slightly bigger footprint. (3GB bare bone config Mac Pros aren't much of an issue because who uses them in that mode? Large majority of those folks are going to add memory to box within minutes/hours/days of getting the minimal config box. Another set doesn't matter because didn't buy it for max performance anyway. )
However, this is a bit of a nudge though aimed at the 64-bit driver development slackers. It is going to be a while before vast majority of Mac Pro users a bumping into this issue. However, doesn't hurt to put the machine into the mode now that will be the mainstream several years from now. By the time more Mac models need to ship 64-bit kernel mode more of the bugs will have been shaken out.
