Not much, but I've never seen anything either way. The higher amount of cache memory, the longer it can hold the burst transfer rate. If you are trying to copy several small files, the larger cache will help speed it up some. If however you are trying to copy large files, both drives will drop down in transfer speed. If you are going to read a file that is 8MBs, then both drives should be the same. (assuming identical drives, just different cache size.) If you are reading something that is 14MBs, then the one with the larger cache will probably be a bit faster. Once you are trying to read a 100MB file, then the difference between them is pretty much nothing.
At least thats how it was with 2MB vs 8Mb drives, if my memory serves.