There are many mounting options. Some mounts are made to be used with a rail, some are made to be used mounted directly to a flat top. Some mounts can be used either on a rail, or, if used with a spacer, on a flat top. If you have a mount that is made to be mounted on a rail, and use it directly to a flat top, the optic will be too low. Some of these rail mounts can be used with a spacer, which will raise the optic to the propper level. Different folks have different prefferences.
I've used both the ARMS #38ex and the GG&G Scout Rail. Great systems, both do what they are supposed to. However, if the Aimpoint is the optic you are using, I dont think they are very necessary.
Forward mounting a dot type optic is important. Some optics arent available with a forward type mount that attatches directly to the flat top. The Reflex is one example of this. For a Trijicon Reflex on a flat top, I would reccomend a scout type rail.
However, as you have seen, there are at least two excellent quality forward mounts for the Aimpoint that go directly to the flat top. These mounts will put the Aimpiont out even slightly further than the scout rail systems will. And both put the Aimpoint at the correct height to be used in conjunction with your iron sights.
I'd be willing to bet that if you call Donnie, he will cut you a deal for a Aimpoint M2, a GG&G Cantilever or Knights Forward mount
AND a flip up rear sight like the ARMS #40 for an excellent price. Im guessing under $650 with shipping (please dont quote me!
). If you skip the ARMS #40 you will be well under $600, easily.
Here's a pic of just such a set up! (all supplied by Donnie BTW!!!