Small chalet companies are often more expensive, with higher standards - you may have to treat it more like a traditional job than a season with them... they're also likely only to offer chalet hosting jobs. The pay might be better than a larger tour op, although that will vary - you can usually pick the resort you want to be in, just by selecting a local chalet company.
Large companies have a range of jobs, and operate in different resorts - but you won't usually get to pick where you're going, just say where you'd like to be; whether you end up there is the call of the company! You meet a lot of people quickly, working for a tour operator, and will often live with a lot of them too - that can get a bit overwhelming after a couple of months, as you basically NEVER get any time away from them, even if you are best mates.
The biggest deal with the larger companies is that you often get an employment package, consisting of the likes of food, transport, lift pass, insurance, and accomodation (housing is REALLY expensive in resort, so this can be a big deal). YOur pay will reflect whatever they give you though - more stuff = less pay, usually. My season was with Mark Warner (well, one of their sub-divisions), and I'd recommend them.
As for resorts - I went to Meribel, which has great nightlife. Courcheval 1850 is nice for skiing but totally dead in the evening, lower resorts in the valley are better (1650 especially). Val is good but expensive, and very British, and Les Deux Alpes is quite studenty from what I've heard. One thing you hear again and again - wherever you end up, you'll have a good time... it's nothing like being in a resort as a punter, as it's home for you, not just a holiday.