Singer is better. This is coming from a fan of Vaughn.
Singer has a better sense of visual style, better use of subtlety, and brought a strong sense of gravity to the dialogue. Some of the scenes in X1 and X2 were classic, memorable, and even beautiful.
I really didn't like the fight scene between Wolverine and Deathstrike though. I get that they would fight dirty, but the way it was shot made it look actually disorganized and ineffective. Hot mess. That said, I would love it if Deathstrike was back. Kelly Hu was awesome.
First Class was hollow and had really bad lines. There were some good things about it, but I was disappointed overall. That "mutant and proud" line was the most horrible thing. It was not subtle at all. Very ridiculous.
I also didn't like the fight scenes in First Class. Can we get some fight scenes that are as exciting as Hit Girl's scenes in Kick-Ass? It can still be good without it being Rated R.