To be as impartial as possible, I'd say that both the Ion and Promaster are fine choices, and if you get a chance, you should try both and see which you prefer (other players at walkons are almost always more than eager to show off their toys and let people have a quick rip on them!) But the Promaster's better
Oh, and it just so happens that I can upgrade Promaster boards to give you all the shooty-shooty modes and fiddle-with-able options you don't normally get on entry-priced guns