QB78 -- uses 2 powerlets (you can shoot with ONE fresh, and one empty, though, with no change in power, just fewer shots*)
QB78D -- upgraded version of the 78, uses 2 powerlets (*same as above)
QB79 -- uses a paintball or AirSource tank (with an adapter),
The QB78D (D for "deluxe") has a nicer stock, and as one of the chaps mentioned, a 15mm OD barrel (the standard 78 has a 14mm barrel, as does the 79, I believe).
The "bling" on the 78D is a gold-tone anodized bolt handle and trigger blade - the trigger internals are identical across the three models.
The QB78D also uses "fiber-optic" or "Tru-Glo" style open sights (...I don't care for 'em), while the standard 78 and the 79 have iron sights. All three have 11mm scope dovetails on a steel breech. If you're going to scope anyway - and you should, they are quite accurate little guns - then the sights aren't that big a deal.
All three versions shoot the same; they share the same valve, striker, and springs. I don't find the 15mm barrels to be any better or worse than the thinner ones.
Both of the 78 versions can be converted to bulk-fill just by changing the cap.
The QB79, designed to run only on external tanks, CAN be used with a powerlet in a pinch, if you buy an adapter (paintball equipment suppliers sell them for around 30-35 bucks.)
And a final note on the QB79's; I have 3 of them (and 2 AR2079's!) and I've never had a leak from any of my tank blocks.
By comparison, the very first QB78 I bought DID leak down in about 20 minutes (bad seal), but none since.
Cheers, mate!
-JC
