I have a SPARC, and a FastFire II. We are encouraged to borrow the departmental products, and I didn't own a night vision compatible optic to use with the borrowed night vision, so the SPARC was a reasonably priced choice.
Nice optic, I didn't use the 2X "doubler" much (at all, actually) because it really narrowed the field of view. My only complaint was that the control "pod" was mighty wide. For NV compatibility in a 1X optic, I don't think you can beat if for the price.
Even though I have an EOTech 512 and an EXPS2-2, my Burris FastFire II sees a lot of use. I have one "mobile showroom" AR, with stuff all over it (an employee discount is a wonderful thing) and I have KISS AR's with just an optic and a light. The FastFire II on the ARF3 mount is narrow, light and just right for that application. When I play with my Adult Lego Set and reconfigure the AR's, I often put the FastFire II on a 45 degree mount, under a scope.
My single complaint about the FastFire II was that the auto-adjust brightness was not as bright as I would like. I lived with it, because while the auto-adjust on the Leupold Deltapoint was much brighter, it was also twice the price of the FastFire II, and I'm a cheap bastard.
Burris addressed this with the FastFire III, which has selectable brightness settings in addition to the auto-adjust.
SPARC:
http://www.opticsplanet.com/vortex-sparc-red-dot-sight-sprc.html
FastFire II:
http://www.opticsplanet.com/burris-fastfire-w-picatinny-mount-4-moa-dot-mat-fastfire-sight.html
ARF3 Mount (current version fits both II and III):
http://www.opticsplanet.com/burris-ar-f3-ar-tactical-flat-top-fastfire-mount.html
FastFire III:
http://http://www.opticsplanet.com/burris-fastfire-iii-red-dot-reflex-sight-w-optional-picatinny-mount.html
Mark H.