for my money, i would go for the larger cache. i use a 34x with 128k cache and it seems for me that is where the bottleneck is. it revs up and down like it's waiting for the info to pass. besides, cd rom speeds are way over-rated anyway, how fast do you really need?
a 32x with 512k will be a better deal in the long run, especially if you lash out and buy a cd copier.
the larger the cache the better i reckon , scsi cd roms have 1meg and some good ones have 2meg buffers, compared to 512k on ide drives.