So with my replacement VP9 FDE coming in on Monday, I wanted to order a threaded barrel for my VP9 so I can run my Osprey 45 on it.

I've done the research, and the prices come out the same.

Silencerco VP9 barrel + 1/2x28 piston + $240-ish

RCM VP9 barrel in 13.5x1 LH = $240-ish.

I would need an extra 1/2x28 piston if I get the SiCO barrel, but I have a spare 13.5x1 piston laying around that I could use with the RCM barrel.

I've heard the RCM barrel finish matches the finish of the stock barrel very well.

Just trying to figure out what makes the RCM barrel almost $100 better than the SICo barrel.

I have the Silencerco barrel, it works just fine with my Octane 45. The finish does not match the original barrel's finish that well, but that's a moot point. The only thing I don't like about it is the barrel is threaded the entire way back to the shoulder, which means it can take a long time to thread on and off. But it feels more secure than barrels that are only partially threaded. YMMV.

I'd say go with whichever one you can get the cheapest.

I have the RCM. I like the 13.5x1

Silencerco just had a great Fathers Day sale. I would imagine they will have a 4th of July sale as well. Hold off and see.

This is the one I put in my VP9.  SiCo Osprey 9 has been purchased and will eventually come home.

https://www.hkparts.net/shop/pc/HK-VP9-9mm-Tactical-Threaded-Barrel-13-5-X1-LH-US-367p16850.htm

Do any of the barrels need to be fitted or are they just a drop in type?

They should all just don't drop in.