From what I've heard the Linn Sizmik will be alright for music but not so good for movies, while the REL Q400 is just the opposite. If you're after a £1000 subwoofer, I would suggest taking a look at the ATC Concept 2 sub (www.atc.gb.net). It's one of the best subs I've heard for music, and does a very good job with movies too, as long as your room's not huge.
As GJC says, this forum is mostly full of Velodyne/Paradigm aficionados - I just thought I'd suggest something you might not have considered.
P.S. - You do have to watch out with the C2, though, as it only has balanced XLR inputs. No high-level connections, or phono sockets, though it's easy enough to get phono-XLR cables.
The REL Q400 has had an excellent review in the past in HiFi World. If it's anything like the Q200, one of which I used to own and adore to bits, then it should be a monster sub quite capable of doing full justice to any movie .1 soundtrack you could throw at it. I suppose it might not be the best for 'music' - you might like to look at a REL Stadium for that, for instance - but I can't see it disapointing. What's your intended set-up and use? As for the Sizmik, again according to this month's HFW there's a new model out of this sub using a 12" driver and, if I understood the article right, it uses digital filtering/equalisation. I spose I'd better head over to Linn's PITA website...
Hmmm ATC subs... yummm... Have to check those out too...
(Of course, you must judge for yourself and not rely on magazine reviews - but they provide useful pointers. I suspect for the money the Q400 would be hard to beat IMO)