Don't trevel on the w/c 16 February if you can avoid it, that's when all the English children are around and prices double.
St Anton itself is quite low (1300m), so has a less reliable snow record. If you are going December or April Val d'Isere is probably safer.
I haven't been to St Anton but know Val d'Isere well and there's a _huge_ amount of skiing there - but it's not a pretty alpine village, it's a ski resort. I agree Tignes is a better base - no need to use the ski bus ever - but Val d'Isere is well linked to Tignes.
If you look at & compare piste maps it's certainly true that most blues in val d'isere would be reds in Switzerland and many of the reds would be blacks - I don't know St Anton so well, but traditionally the espace killy grading is the hardest in Europe.
I've been skiing & on holiday in both countries so you could consider other factors:
If you speak French or German to any extent, go to that country. Everyone in the resort will speak English but you make more friends if you talk the language.
If you like cheese, go to Val d'Isere. If you like dumplings (knudeln) go to Austria. Actually, France always beats Austria where dedication to food and wine is concerned.
The French like long lunch breaks. There are never any queues between 12 and 2.
For details on historic snow records, a summary of the resort etc look at www.skiclub.co.uk
(go to historic snow reports then click on resort info for more) - very good for all European resorts.