Re: What would you rather see next - FIRST CLASS 2 or X4?
As names/concepts without any detail, either sound fine.
If I knew more about who/what might be included in them, then I could choose!
X4 would seem likely to omit Cyclops, Jean, Xavier, Mystique and Magneto, and possibly Rogue as well. That would leave Storm, Wolverine and whoever else they decided on including (possibly a new team altogether, as at the end of the Last Stand novelisation). Of course, that's only my quick assessment - they might well bring back some of the dead or cured characters for all we know.
First Class has many decades to fill in yet, and the younger, more vibrant cast does seem a fresher option overall.
Honestly, I can't choose, but I think a First Class sequel would be easier to get off the ground soon - they have the cast, they have the locations, they have a director wanting to come back.