Which shuld i choose MOD WISE?

Mmm. Thats a very good question. There is literally fantastic mods for each game.

Id say both.

Isn't there a mod for New Vegas that allows you to literally connect the two titles into one world?
I believe that mod is based off New Vegas, though obviously it requires both titles. Having said that, if you're unsure, I'd start with New Vegas, as I believe that mod can allow you to use your NV character in FO3 and actually complete FO3 mods with him. I do NOT think it works vice versa, AKA you make a FO3 character and then use him to explore New Vegas mods.

I'd say you might want to decide based on which vanilla game you prefer. The best argument I can offer to go with NV is that the modding community is still reasonably active with new mods being released regularly (not tons of them, but enough to keep the game interesting). I have a fully modded FO3 game that I've had for years, but it's not the same exciting community it once was, that's for sure.

Wait are u wanting to make the mods urself for the game, or deciding which games based on mods?