Hard to tell, just from the info you gave. I have run in Kinvaras, have done a couple half-marathons in them and I really like them. But based on the half-marathons, I know they wouldn't work for a full. You really can only go by your experience. How soon is the marathon? Can you get in a long run (16+) in the Merrells? I personally think it's around 16 - 18 that you really start to feel any shortcomings in the shoes. You don't say which Asics you bought and it doesn't sound like you gave them much of a test.
Also interesting that you sometimes hit 50+ (with no long run) while typically running 20 - 30. That's a big difference. But that's another matter.