A few universal truths:

91/119/135
1. Almost always live in you base.
2. Bases don't change too often
3. Career pay is flatter (starts higher but doesn't increase as much)
4. YOU file flight plans. YOU make all the decisions.
5. Very limited travel benefits
6. May never had "hard" days off
7. If you are flying two pilot, you will probably fly with the same person (or 2-3 people) your whole time there.

121
1. Live wherever you want, but deal with the commute.
2. Bases can open and close fairly quickly
3. Pay starts crappy. If you hit the lotto and get on with a mainline and spend 20+ years there, the pay gets pretty good.
4. Dispatchers (or rather the computers they use) calculate and file flight plans.
5. Good travel benefits
6. Have a minimum of 10 days off (at my company-I think the lowest). Not bad if you live in base, but, depending on your commute, that 10 days could turn into a quick slap and tickle with the Mrs. while doing your laundry.
7. You can bid to avoid that one "jerk" captain.


Having done both, I prefer:

the schedules of 121
the flying of 135
the smaller companies of 91/135

Good luck