Basically, the CPU is used for the data transfers, as opposed to a processor on the SCSI board.
As far as a Z2 card goes, there's not going to be much difference in performance when it comes to raw transfer rate. Multitasking would be nicer with a DMA card, though, since the CPU is mostly free while the transfers occur.
To get a better idea, try
this benchmark on both and compare the results.
