Well you can gain muscle mass either way. It depends more on what your going for...endurance or greater strength. If your going for endurance I would do more reps. If your just going for strength than do more weight but fewer reps. I find that if you use both methods you can get the best of both worlds.
Usually when I work out I go for endurance (more reps lighter weight). I lift 3 times a week and 2 times I lift lighter weight w/ more reps, but on the other day I put on more weight and do fewer reps. I found it worked for me as far as gaining muscle mass. You might want to try something similiar.
Oh and as far as how many sets/reps/weight to do...well that depends on how much time your have, but my weightlifting usually only takes me about an hour and a half. Right now here's what my workout looks like...
Monday and Saturday: (endurance days)
BenchPress: 6 sets of 10 96lbs
Curl: 6 sets of 10 64 lbs
Back Press: 6 sets of 10 64 lbs
Handstand pushups 6 sets of 7
Regular Pushups 6 sets of 30
Upright rows 6 sets of 10 64 lbs
Bent rows 6 sets of 10 64 lbs
Thursday: (heavy day)
BenchPress: 6 sets of 6 126 lbs
Curl: 6 sets of 6 84 lbs
Back Press: 6 sets of 6 84 lbs
Handstand Pushups 6 sets of 4
Regular Pushups 6 sets of 20
Upright Rows 6 sets of 6 84 lbs
Bent Rows 6 sets of 6 84 lbs
Now I'm not saying you have to do exactly the same thing that I did, but it's what I do. I'm in Tae Kwon Do so I don't really want to bulk up, I want strength but speed and endurance as well. It has given me lean muscle mass and I can see my arms and chest getting bigger.
When I first started out I would only do maybe 2 or 3 sets of this stuff. Then when I was comfortable with that I would move up to 4 sets and so on.
8-10 reps is best for gaining mass. Larger muscle groups need 6-8 sets, and smaller muscle groups need 1-3 sets. I've done way more than that, but I wouldn't recommend doing more. Every 3 weeks change up your routine. You can increase the weight, take shorter rests, or do more sets. Just keep your muscles guessing. Hope this helps you out.
...Curl: 6 sets of 10 64 lbs...
...Curl: 6 sets of 6 84 lbs...
I guess I'm confused about this on two counts. One, normally when I see a persons weight I see it in dumb bell amounts. I can't imagine you'd be doing 64 lb dumb bells. Two, I assume its halved or you use a machine or some sort of curlie bar. But still, how in the world do you get 64 lbs? I guess theres 2-25 lbs, two-five lbs, & 2-2lbs...am I right?
The reason I find it odd is like I mentioned, always have dumb bells on the mind. All the ones I use increase in 5 lb increments which I think is great. I liked what you had to say about lower weights and higher reps though. I do that sometimes. So like heavy days for curls I will use 50-55 lb dumb bells and lighter days I may use 40-45 lb dumb bells. I know it may not seem like a big diff but pushing yourself is good now and again.