The short answer
Choose Val d'Isère for a family trip where everyone skis well, or if you're keen skiers who also want good evenings. It's a real village, much of it is ski-in, ski-out, and it has the best night-time scene in the area, with clubs and parties.
Choose Tignes if you're a party or young group, if some of you are beginners, or if you're watching the budget. It's lively and fun, it has more apartment complexes and so more cheaper options, and it has a few easier runs, including easy ways home.
Either way you ski the same area. The two resorts sit on either side of a ridge and link in 1 or 2 lifts, so the choice is about evenings, budget and the run home, not about the skiing.
