One thing bothers me, is the any difference between the bioses???
Any extra speed using new bios? don't think though.
Only thing I know that PSX with OLD bios can play backups perfectly without MOD-CHIP...
The only other difference was the ones prior to the 7001 (I believe) didn't have analog or dual shock support. But I think that was the console itself and not the bios. I can't say I've seen much difference between running the 1001, 7001, or 7502, so I just stick with 7502.
The difference is the region of origin. 1001 is from a us machine, 7502 is from a european (pal) one. I suppose it would be better to use the bios from the region that the game came from for compatibility reasons however in general the difference is irrelevant.
I use the 7502 bios and yes there are some differences (though minor as CD said) besides being from different regions as Betamax correctly stated. Proof of these differences is that at least one game, Wild Arms 2, requires the 7502 bios to run at all w/ePSXe. I have all the bioses and I tried all of them w/this game and the only one that it would work with was the 7502 one. So there are differences, no matter how slight, between the different bioses.