I'm sure you're all aware that a float tube is actually a replacement for the barrel nut. That is, it is a really long barrel nut. A DCM-legal float tube is a really long barrel nut with a front sling swivel at its outer end. Given that function, there are several considerations in selecting a float tube. Primary among them are the means by which the float tube is secured, and the allied means by which the tube may be properly aligned.
Some float tubes, those intended for "space guns," simply screw into the upper receiver to retain the barrel and replace the barrel nut. They have no means of indexing, nor is one needed as they are symmetric and round. A second type of tube is the two-piece tube, where an extended, threaded "barrel nut" is screwed into the upper to retain the barrel, and a float tube is in turn screwed onto it. The tube may be indexed in rotation, should there be some assymetry to consider. The tube itself is secured by LockTite to the extended barrel nut. However, there is no mechanical means of taking up thread clearances or securing the tube coaxially with the barrel (they tend to sag at the free end). A variation on this theme uses set screws to supplement the LockTite. This type of tube permits indexing but still is not particularly stable.
Most sophisticated are the three piece tubes, where the tube is secured by means of a lock nut to the "extended barrel nut" after it is indexed (LocTite is often used as a supplement also). This arrangement provides both indexing and security.
As the DCM-type float tube must be indexible (unless you want your sling swivel pointing to the side) and very secure, the three piece is the only way to go. You may pay a little more for the Fulton Armory assemblies, but you should accept no substitutes.
For installation, please take into consideration the cost of a complete job, including modifying and properly installing the handguards. A seemingly low-cost installation can escalate in price rapidly when the "mandatory options" are added in! Fulton Armory's policy is to do work such as this only in conjunction with a Tech Inspection. When you receive your rifle back from Fulton Armory you may be assured you have a complete and prepared arm ready for service. There'll be no "why didn't you notice the front sight was on backwards???" answered by "you only asked me to attach the sling!" Oh, and if your new rifle is shipped as my Bushmaster was, it will be in dire need of proper lubrication! There's nothing like steel on aluminum to age an AR very rapidly!
Hope this gives some ammunition for your consideration of the alternatives.
Walt Kuleck
Addendum from Clint:
Yes, the float tube is an essential addition to the M16/AR15 rifle for accuracy. This is been quite understood for several years now. The good ones are expensive, so you'll hear arguments as to why you don't need'em. But you do.
Clint McKee