The Bushmaster with the Izzy flash hider installed is likely a 14.5" barrel with the flash hider permanently installed. As you noted, you can always buy a birdcage and replace it later with whatever you like on a 16" barrel.
Also, is there any difference, other than the stamped horse, between Bushmasters and the higher priced Colts?
Colt proof tests and magnetic particle inspects its bolts and barrels. Bushmaster magnetic particle tests individual barrels out of each run and does no proof testing.
A Colt M4 will use M4 feed ramps and an M4 barrel extension.
The Bushmaster will use standard AR feed ramps and barrel extension.
A Colt M4 will use an F-marked front sight base that is slightly taller to compensate for the different geometry in a carbine.
The Bushmaster will use the standard AR sight base that a 20" uses.
A Colt M4 will have a heavy buffer *H buffer"
The Bushmaster M4 will have the standard carbine buffer
The Colt M4 will use a military spec buffer tube extension.
The Bushmaster will use a buffer tube extension that is slightly smaller in outer diameter.
The Colt M4 will have double-heat shield handguards
The Bushmaster will have a single heat-shield handguard (I actually prefer this)
The Colt will have the nut on the telestock staked
The Bushmaster will not
The Colt will have the sling swivel mounted on the side of the front sight base
The Bushmaster will have the sling swivel mounted in the traditional location.
Besides these two differences, the rifles will be pretty similar. I've owned several Bushmaster rifles over the years and always been pleased with them. There are important differences in QC between Colt and Bushmaster; but I think the Bushmaster still represents a good value if you don't absolutely need those things or plan to make a lot of modifications anyway.