[soupah]

If I understand correctly, the Colt .38 Trooper (not MKIII) and Colt Official Police look alike, share the same frame and chambering, so what's the difference? Is it smaller things like the grip, sights and hammer? Does the finishing or internal lockwork differ?

More to the point, I have two Colt .38 Troopers...is it worth getting the Official Police (other than the worth of having a more complete collection)?

I have a line on one for $300, but I'm not sure it's worth buying if I have already have, in essence, two other Colts exactly like it.

[Texas Colt]

The Trooper has adjustable sights and the OP has fixed sights. They both use the same frame, lock work, and grips.

[dfariswheel]

Up until about 1961 or so, the Trooper .38 had the firing pin on the hammer, just like the Official Police.
After then, the .38 and .357 Trooper had the firing pin in the frame, like the Python, but the OP retained the hammer mounted firing pin.

The Official Police, Trooper, Python, 357, and Officer's Model are all the same basic gun.
The original Trooper is essentially the Official Police with adjustable sights, and options for a Target hammer and grips.

[SwampWolf]

I have a line on one for $300, but I'm not sure it's worth buying if I have already have, in essence, two other Colts exactly like it.

You may have two already and far be it from me to spend your money but, imo, if that Colt's in nice condition, $300.00 sounds pretty cheap.