I would definitely wait. The myTouch 4G is a great phone, but it seems like the 2X is too. Dual-core does make a big difference so that definitely is a major plus. But personally, I think the UI on the 2X is terrible, that's just me though. I really like the myTouch 4G's UI. I can't say which is a better phone for two reasons. 1, its not out yet. 2, everyone could tell you "oh get the myTouch 4G its amazing!" (Which it is), but it comes down to preference, your opinion. Each phone has its benefits.
So I would say to wait till it comes out and play with them both in a T-Mobile store and see which you like better. Definitely don't jump the gun on it yet, wait.