The SR-25 is more of a "custom" rifle.  Lots of cool parts (such as the RAS and flip-up sights) are standard.  They also use match barrels exclusively.  There are some problems, though:

- They use original Armalite waffle AR10 mags, and pre-ban versions are extremely rare and expensive.  Not an issue for the military or LEOs, but a big one for the rest of us.
- Many of the less visible parts are somewhat poorly designed/engineered.  Armalite has a good FAQ on the differences between the two that point out some of these.  The Navy had some SR-25s for testing, but has returned most of them due to problems related to some of these items.
- Getting support for these rifles is difficult for non-military folks, as Knights is primarily a manufacturer to the military, and focuses there.
- Price: custom rifles have custom price tags.

The Armalite (the current version, in Genesco, IL) AR10 was somewhat based on the SR25, though with some significant differences, the main one being that it uses modified M14 mags, which are much more plentiful.  It is also more of a production rifle, so parts are a bit more consistant, spare parts are readily available, and there are mulitple sources for upper halfs in virtually any configuration, in addition to the wider range of factory options.  Finally, lowers are available, allowing you to build up any desired configuration without having to discard expensive but unwanted parts.

Pros:
- Much larger number in civilian hands means wider support base.
- Mags much more plentiful and less expensive than SR25.  Mags also more rugged.
- Chrome-lined "military"-type barrels offered.
- Much lower cost.
- Much wider availability.
- Excellent customer service, readily accessable to civilians, lifetime warranty on complete Armalite-branded rifles.


Cons:

- Accuracy isn't as consistant across models (most models do not use match barrels).
- No "high-speed" items like RAS or flip-up sights from the factory, though aftermarket options exist.

Basically, I'd only go with a Knights if I was looking for a collector.  Being both rare on the civilian market and a supplier of "cool stuff" to the military, they will always have high resale.

On the other hand, if you want a shooter, Armalite has most, if not all, of the advantages.

-Troy

Armalite has a lot of info in their FAQ and Tech Notes sections.  And here's a pictorial at BiggerHammer.net:

www.biggerhammer.net/stoner/sr25var10/

-Troy