are you going for muscle endurance? (like training for long-distance or long duration sports)
or do you just want to look as big as possible?
if yes to the prior, less weight more reps (or the same weight more reps)
if yes to the latter, more weight less reps (or the same number of reps just add weight)
in general i've just stuck with 10 reps per set and kept adding weight when i needed to... it's a good number thats in between bodybuilding level and lean muscle building level. the guy who helped me figure out a routine body-builds professionally with arnold schwarzenegger and during his peak he would do 3 reps per set and an 8 minute rest in between sets. anything upwards of 12 reps per set is training for muscle endurance and while it WILL help cut away at fat and build you a lean physique/make you stronger, it won't make you look as big as doing less reps with more weight will.
