Been to both. Les arcs for me. if you want rugged, technical mounatin singletrack then it has to be les arcs. If you prefer man made DH tracks and play areas then Morzine/Les Gets is the place to go, thats not to say that Morzine doesn't have mounatin singletrack as it does, just not the same quality that is found in Les Arcs IMO. Le varda is a must trail in Les Arcs, 45 mins of rock infested, swithback laden, steep, technical mountain singletrack.
Les Arcs is much quieter as well, no crowded lifts and the trails mostly to yourself (at least thats how it feels). Morzine has a better nightlife though if thats what your after.
To get the best out of Les Arcs you'll probably need a guide though. Trail Addiction are very good.
Which ever you choose you'll have fun.