If you make a new project, I'd recommend starting in MZ. VX Ace is great, but it's also aging and on some systems it's already stopping to work correctly. MZ can be changed to support a tilesize of 64x64 with a plugin, so you can theoretically use Ace's assets (which are intended for 32x32) since the scale factor is exactly 2, so there wouldn't be any quality loss.
But all that said, if you have a longterm project already going that's in Ace, I'd instead recommend trying to finish it since porting can be quite the challenge.