as far as i've read, bands are much better for the squat and chains are better for the bench. personal experience says that bands are better though i cycle the bands in and out of my benching to let the elbows get a break. i use 5/8" chains and they are 40lbs per set.
whether it is enough weight or not is totally subjective. this depends entirely on your strength level. do you think an extra 40lbs is going to mean alot to an 800lb squatter on speed days on top of his normal weight? i would say probably not. you have to use whatever is correct for your strength.
here is the general guideline for accommodating resistance using chains as per strength level:
Max Squat
Chains per side
Weight of chain (top)
200-400
1 (5/8)
40
400-500
1 (5/8), 1(1/2)
60
500-600
2 (5/8)
80
700-800
2 (5/8), 1(1/2)
100
800-900
3 (5/8)
120
here is the listing for bench:
Max Bench
Chains per side
Weight of chain (top)
100-200
1 (1/2)
20
300-400
1 (5/8)
40
400-500
1 (5/8), 1 (1/2)
60
500-600
2 (5/8)
80
600-700
2 (5/8), 1 (1/2)
100
none of this of course is set into stone but you might give it a whirl. hope this helps.