I think any one of the three would be solid - I have 2 of the BCM variants with another inbound as we speak. (ETA: they work fine - love them)
I would say that given ‘price and availability isn’t an issue’ I’d probably go with the Vltor - for no other reason than their reputation and the fact they had the idea and designed the chopped rifle buffer which is the A5.
I’ve used many A5 RE tubes. The two brands I use that have the best fit and finish hands down are Vltor and Magpul. The Magpul SR25/M110. RE Tube is a 10 position A5 tube. I actually run more of those than 7 position Vltor’s and they Are rarely out of stock.
Out of the VLTOR and the BCM, I prefer the BCM. Better fit to stocks and I think the anodizing looks better. I don’t have any experience with the others.
I have used both brand new (non-blem) BCM and VLTOR. No quantifiable reason, but I prefer BCM - I think it’s a little bit nicer than VLTOR cosmetically. Plus BCM has 8 positions which is also a minor plus IMHO.
VLTOR blem unit can be bought a bit cheaper than BCM: https://www.vltor.com/shop/specials-and-sales/vn-re-a5-a5-receiver-extension/
I’ve used Vltor and BCM, nothing negative with either. I’m just glad we have three quality choices now and can usually find at least one of them somewhere.
Just a quick update, since I have all three of these A5 receiver extensions now.
In a ranking of best to worst.
#1 = Vltor
#2 = BCM
#3 = Magpul
This is only a ranking based on their finished appearance, as they all should work equally as well.
The Vltor and BCM receiver extensions are almost equally as nice in their external finish, with the slight edge going to Vltor. Internally Vltor wins as it’s interior finish is mirror smooth, the BCM is a little rough with visible swirls/machine marks. The Magpul receiver extensions are rough looking externally with almost a cracking look to their surface texture, and they are just a tad worse looking internally than the BCM receiver extension.
The Vltor receiver extensions are basically almost equal to the internal smoothness seen in the old stock Colt rifle receiver extensions available at Specialized Armament.
In my experience I’ve not experienced any finish issues with Magpul. I don’t know exactly how many Magpul RE tubes I’ve used but the total number across all A5’s is 20+. I’d say the finish was better than the 3-4 BCM’s I’ve used. The fact mine get marred, scratched, some painted means that it really doesn’t matter aesthetically. That’s the only differences I’ve seen.