I've noticed that both へ and に are use with ようこそ. The one I learned first is へ, and it seems like it's more common, but I know に is also used, as in the title NHKにようこそ.
When I search BCCWJ, I find that へ is more common, at least when it comes right before ようこそ:
へようこそ 69 results
にようこそ 37 results
Of course, this is a written corpus and it might be different in speech. But assuming I'm right that へ is generally more common, are there any other differences between the two? I always say へ, but every time I hear に I wonder if there's a difference in register or such that I'm not picking up on, or if they're totally the same.