The wedding invitations look great! LidaB is right that the heavier the stock, the better it will hold up- especially when embossing with heat. Vellum does tend to curl easily from heat because it is made with a substantial amount of plastic.
Anyway, as for the weight- actually all those weights are fairly thick for translucent vellum stock. Lots of translucent stock is down under 40lb. Honestly, for your project, any one of those above 60lb should work great!