Well, personally, if I had to do only one, it would be the SIR and Aimpoint. But thats just me. Here's why;
First, I like the Aimpoint much better than any EoTech. I have always found the Eotech reiticle to be somewhat grainy. Battery life on the EoTech is also pittiful when compared to the Aimpoint. I do not like the button controls of the EoTech when compared to the simple knob of the Aimpoint. I think the aimpoint is just a simpler and more robust optic. I think it is built tougher and more durable. Having said that, I know too many good people who love their EoTech's and I know they are a fine sight. In addition, EoTech should be releasing a new series sight soon that addresses the few complaints there have been. I look forward to seeing it.
Also, please understand I do not actually have my RAS II yet. I just tracked it via FedEx and it will be delivered to me this week. However, like I said, I have been following it since any of us first heard about it many months ago. Based on what I have actually seen and done with the SIR and what I know about the RAS II, I believe I will still preffer the SIR over the RAS II. Here's why.
I like the fact that the SIR has removable rails. Rails are good thing to have on your handguards. But unwanted rails are uncomfortable to hold on to and they can heat up very quickly. The RAS II's answer to this is to cover the rails with pannels. Pannels are ok, and they do sheild your hand from the edges of the rails and the heat they will acquire. But the pannels are directly attatched to these aluminum rails and even they will heat up eventually. The SIRs answer to unwanted rails is to allow you to take them off completely. I like that. You dont have to worry about having or losing pannels, or having the pannels heat up. You dont need the rail on that part of the gun, take it off.
I also like that the lower half of the SIR is synthetic, not aluminum. I am telling you, the lower portion of the SIR simply does not heat up. Period. In some testing I did, I ripped 5 mags through the SIR on full auto as fast as I could load the mags. It got warm, but no where near to hot to hold. I did the same thing to an Oly FIRSH which is another aluminum rail handguard, and it became too hot to handle. If all goes well, I will be doing the same test to the RAS II I have coming on Sunday. We used a digital thermometer in these tests and will have precise figures to show people just how hot they got.
In the end, I can hear someone saying that the synthetic lower of the SIR is weak and they would rather have an all aluminum handguard. I disagree, but not so strongly I dont want an RAS II myself! I can also hear people saying they would rather lose the removable pannel of the RAS II than the aluminum rail of the SIR. Again, I can see the point, but the off chance that I lose the actual rail does not outweigh the advantage of not needing pannels to begin with that I get every time I shoot the gun. Others might disagee and I dont feel strongly enough about it to make an argument.
In the end, either would be a great set up. Thats why I am getting both! But my favorite AR, the one that gets all the trigger time and attention has all my favorite gear on it. Needless to say, it has an SIR.