FWIW, those 2.5 gallon kegs you're seeing at Northern Brewer are not true "corny kegs" any more than Torpedo kegs are. Both are modern knockoff variants of corny style kegs, which Cornelius/Firestone/etc. haven't made in a long time.
I haven't used Torpedo kegs myself, but if stackability is something you need, they would seem to be a no brainer.
I have used various Italian and Chinese corny knockoffs. If you go that route, I highly recommend getting kegs with rubberized handles. The bare metal "strap" handle versions are a pain (literally) to lift and carry.
I actually obtained 4 "real" Cornelius 2½ gallon kegs about 15 years ago, $150 for all four, in pristine condition. Fella had 'em and didn't use them. I was just getting into kegging and thought, "What the heck. Why not." They get used quite a bit when I'm half way down in a 5 gallon keg and need the extra keg space for a new batch, and they are still the cleanest "used' kegs in my collection.
I also have two 1.6 gallon (6L) Torpedo kegs I got as a Christmas gift from my son a few years ago, who expects that I'll fill them occasionally for HIS kegerator on a recurring basis. About the same time I also picked up a 2½ gallon Torpedo keg. They work fine, though initially I had issues with them loosing pressure, mostly around the keg lid seal, but also at the post seal gaskets. That was shortly after they first came out in the market. I haven't heard many complaints about poor seals lately. It may have been that the lids and post seals just needed to "set" against pressure. And I'm more careful about tightening the posts when I clean or replace seal gaskets now. I haven't had any pressure leakings lately.
The stackability of the kegs is definitely a plus, and there are no issues with the rolled steel handles like what
@VikeMan mentioned with the Chinese knockoffs. The 1.6 gallon kegs are also useful for cleaning kegerator lines and taps without wasting lots of CO2, or PBW and BLC. Very portable for a trip to the beach or a camping trip as well. They fit nicely inside an Igloo cooler with plenty of room for ice, a pony tap and a min-regulator and 70 gram CO2 cartridge. Even the 2½ gallon one will fit (barely though) if you have thirsty friends.