I skied Tignes in May 2008- another good snow year that saw skiing back down to the ~2000 m village. They did not have the top of the glacier open the day I went because of some wind (which was very light lower down). Skiing was very enjoyable spring conditions with sunny skies- not overly firm when I first got out and quite soft in the afternoon. Had the top been open, it would have been almost 5 K of vert available. They had good early snow this year and a bit of a top of recently, so it MIGHT be similar. I think the link to Val d`Isere closes May 1, so you will be missing that.
For me, it was only about a half hour trip up the road as I was living in Bourg St Maurice at the time. If you have never skied the Alps before, it might be worth the hassle to take boots and one spring outfit just to get an idea of the size of these resorts. Then again, the skiing that time of year is more limited than earlier in the season and you never know exactly what conditions will be like. If weather moves in above tree line (most of Tignes), then you really can`t see much.
Tignes is a safer bet for skiing because of the glacier, but I believe May 5 is the end of the Dutch week in Val Thorens, so that could be fun as well.