Monitor Audio FB210 or REL Strata ?

You'll need to specify the cutoff of your main speakers and the size of the room you're driving. If your main speakers are full range then make sure whatever sub you buy has a crossover setting adjustable well below the mains. Also, if your room is large, or you like to shake the pictures off the walls you might want to add the ACI titan to your list. It is apparently slightly more difficult to integrate than the strata, but has much higher output at less cost.

I own a strata3 ... it's very good, but has limited power. It's great strength is the low crossover settings which allow me to cross with full range (but limited very low bass) main speakers without any mid-bass boom. However I use mine for music only, and it never shakes my small listening room ... very musical, but not earth shaking.

Is the RELQ201 outside of your price range ... I hear that's a better bet for movies, though I have not heard it.

For a slightly cheaper sub the Rega vulcan (designed by REL) is getting great reviews in the UK, land of REL.

I think I'd go with the REL on that basis because the FB210 doesn't appear to give a significant frequency extension over your main speakers, and may give a nasty midbass hump because of the crossover being set too high.
If you're unable to demo the subs get the REL strata ... it's most likely to work for you. If you can demo them all then let your ears decide ... perhaps the FB210 will save some money.
I think buying the FB210 without demoing both it and the REL would be extremely risky. REL has a good reputation for a good reason.
Why not look for a nearly new REL strata for around $900 ? That's the best of both worlds.

Also, consider resale ... if this is a possibility then again this favors REL ... they usually resell within a day on Agon. The FB210 might be good, but I think it would be much harder to resell.

If you haven't heard a quality sub in your stereo I think you're in for a treat. My strata transformed my CD collection. Some of the live CDs and large orchestral pieces are so transformed I'd find it hard to go back to no sub. Have fun !