The thing with watching videos on a classic is that youre going to have to convert files to watch with good results regardless of which 680x0 cpu youre using. If you take it to the nth degree you'll get away with even an '030 + fast ram so long as you have enough storage space. Obviously a faster cpu has more chance of coping with industry standard formats (maybe mpeg1 for low res video will be ok on an overclocked '060), but end of the day its not the same as watching videos on a modern mac/pc/console/etc., you pretty much need to "amiga-fy" the video anyway.
The '060 will be faster, but due to the need to convert videos anyway, it'll be more of a case of the need to less "optimising" the video needed than better results.