I have owned a 6.2 and a 6.9L concurrently (had the 6.2L Blazer for 8 of the 26 years I've owed the 6.9L). I have little hands-on experience with 6.5L engines, so I won't comment much on them, but from all I know second hand, in most ways, the the 6.5L is a much better and more evolved powerplant than the 6.2L and the last of them are pretty darn good engines. Not powerhouses compared to the other more modern engines, but taken about as far as they can be. I've not owned a 7.3L either... not that they are all that much different than a 6.9L
The 6.2L and 6.9L each have their good and bad years in their eras, good and bad points. NA to NA the 6.9L is the clear winner in power and performance (135/240 vs 170/315 pwr & torque). Conversely, the 6.2L is the fuel economy winner but the gap is very narrow if you focus on equivalent GVW trucks (i.e. an 6.2L in a 3/4-ton vs a 6.9L in a 3/4-ton). The power difference is also stark in that category. A 3/4-ton at a full load with a 6.2L is a pig compared to a 6.9L truck at the same load. Overall, the 6.9L is the durability winner as well. You can't really flog an NA 6.2L that hard (vs a 6.9L) before something gives and flogging is pretty much what you have to do to an NA 6.2L in a 3/4 or 1-ton platform that works. IMO, the 6.2L is a GREAT light duty powerplant, perfect for the half-ton realm. It's more compact and lighter than the 6.9L... basically it will fit anywhere a BB Chev will fit. The 6.2 responds well to a turbo and is very reliable at a 200hp/400 lbs-ft flywheel level. It's at the same place a 6.9L is at 250hp/500 lbs-ft flywheel. I came to the odd conclusion that a 6.2L with a turbo is MORE reliable than a 6.2L NA because you don't have to flog it all the time to obtain decent performance (200 hp vs 135 would kinda do that). I really think the same is true with a 6.9L.
Anyway, if you are trying to decide on a 6.2L vs a 6.9/7.3L, if you are talking older truck in the IDI era and you want a 3/4-1-ton, get teh Ford unit. A really sharp 6.2L GM with a turbo... I wouldn't automatically walk away from. I wouldn't advise getting a 6.2L NA in a 3/4-ton or up unless you also had the coin to buy a turbo kit immediately. A later 6.5L turbo GM would be OK. They had pump glitches with the DS4 electronic pump, but there are good cures for them now. A 6.5L has less power than a 7.3L PSD, however.