Depends on what you do. I've tried both and never noticed a difference. Maybe if you work with many small files, the extra cache would come in handy, but other than that, usually other things will limit you more than the HD cache. Most of the time I'm transferring files 4-6 gb in size and there at least it has not made a difference.
That said if they both cost the same, might as well buy the one with the bigger cache. But if there's something like a $20 difference I would personally just buy a larger capacity drive than one with more cache.
This HDD would be exclusively for the OS (Win 7 64-bit) and programs. There will be no personal data files on this HDD.
Currently, the space used for the OS and programs is about 146 GB. The HDD in use now is Seagate Barracuda ST3750525AS 750GB 7200 RPM 32MB Cache SATA 6.0Gb/s 3.5" (overkill, but it was laying around).
The smallest HDD with 6GBs is 500 GB. And if I want 64 MB Cache, then the smallest available HDD is 640 GB.
This will be used exclusively for the OS and programs. Common sense tells me that 64 MB Cache is better than 32 MB Cache, but my question is how much of a performance improvement will I see? Ignore price differences.