The Reflex is a great sight. As you know, it has no magnification, so technically, it is not a scope. However, it is a tough optic and works well in most conditions.
However, it does have some serious issues with reticle wash out. Because it is powered by fiber optics and tritium, it is dependant on having either good light, or no light to work. If you are in a dark area, looking into a bright area, the reticle washes out easily.
This is a big problem for some folks. I have had a Reflex for 3 years now and though I love the optic, it remains a big problem for me.
I think the best way to mount any non magnified dot optic is to forward mount it. There are only a few good ways to forward mount a Reflex, and I really think the best of these is on the carry handle mount. There are other ways to do it, like with an ARMS #38-ex rail and a ARMS #15 low profile mount, but I find the carry handle mount a better option. I have used both.
In conclusion, I would recommendthe new Aimpoint M2 over the Reflex. I dont dislike the Reflex really, I just think the Aimpoint has an important edge over the Reflex. Plus with an Aimpoint, there are a lot of great mounting options. My recomendation would be a GG&G Cantiver mount or a KAC Offset mount right to the last rail of your flat top.