Here's the list:
The Remington Factory is the best.
The extension is expensive and can be hard to find.
Now that Remington is installing them on the Express gun, I think you can buy them direct from Remington.
Most any dealer can order you a police version from a Remington Law Enforcement distributor, IF he wants to. Some local FFLs just don't want to.
The factory barrel clamp is the best, and interlocks with a groove in the extension tube, preventing the clamp from moving under recoil.
Choate, available from Choate or Brownell's, is the "best buy".
It's high quality, proven over 30 years, and comes in blue-black sulfate, or parkerized.
It come in lengths from 7 shot to 10 shot, and comes as a complete kit with the tube, spring, heavy duty follower, sling swivel, and a barrel clamp.
www.riflestock.com/
www.brownells.com/aspx/NS/store/ProductDetail.aspx?p=4203&title=SHOTGUN+MAGAZINE+EXTENSION
Tac-Star.
Tac-Star is a lesser quality then any other steel unit.
They have a past history of quality problems including egg-shaped, out-of-round tubes and mounting collars brazed on crooked, making the extension out-of-line.
Does not normally come with a clamp.
www.brownells.com/aspx/NS/store/productdetail.aspx?p=13101&s=
Wilson/Scattergun is good.
Quality is good, but it doesn't come with a clamp.
Some people don't like the sling swivel setup.
www.wilsoncombat.com/index_sgt.htm
Hans Vang.
Very good quality, but expensive. Does not come with a clamp.
https://www.vangcomp.com/
Tank's Rifle Shop.
I personally rate this one as second only to the factory unit.
A direct copy of the Remington factory minus the extended cap for mounting a bayonet, and doesn't have the interlock groove in the tube to lock the clamp in place like the factory version.
VERY good and high quality.
www.tanksrifleshop.com/remshotgun.htm
ATI plastic.
Junk, but if it's all you can afford or find it's OK.
Couple of points on the clamp.
It doesn't matter HOW strong the extension is, it can be a solid steel bar.
The extension is screwed to the GUN's magazine tube, and the deep, square threads of the magazine tube weaken an already thin tube.
An unsupported extension is a handy lever to be used to break the extension off the gun.
All it takes is a bump.
This invariably damages the gun's magazine threads, and this means an expensive trip back to the factory to have a new one brazed in and the receiver refinished.
USE A CLAMP. There's NO valid reason not to use one.
If your extension does not come with a clamp, the best aftermarket is the Uncle Mike's direct copy of the Remington Factory.
It's doesn't interlock with the tube, but due to the design, it does stay put better than any other type.
www.unclemikes.com/adtemplate.asp?invky=9004490&catky=1465723&subcatky1=4548843&subcatky2=2000180
My "best buy" is the Choate extension kit due to the combination of quality/reputation/price/finish. It's easy to get, and is more affordable.
It's a complete kit, and they offer many lengths and make a version for most guns.