Since you're a lefty, I'm assuming you'd prefer a lefty-friendly lock, so to me that would rule out liner locks and framelocks since the Military is the only model I know of that comes in a lefty version. (And technically, I don't see it in the print catalog, so I'm not actually sure if it's still in production...?)
Another question is whether you're looking for a knife similar in size to your Delica. If so, the Native 5 is pretty close, but with a *slightly* smaller handle and a tiny bit less cutting edge due to the finger choil. (Although also thanks to the choil, the handle *feels* bigger in hand than the Delica to me, which I consider a good tradeoff.) Overall, the N5 is fantastic. I love mine and can't imagine you'd be disappointed. It's definitely a significant upgrade from a Delica.
Caly 3 or 3.5 are also excellent choices in lockbacks, plus they feature the best clips Spyderco makes, in my opinion.
Another lefty-friendly choice that happens to be my overall favorite is the Sage 3 (Bolt Action lock). I think the SFO still has the Blue G-10 version in stock, which is listed at $209. (A lot of people prefer the CF version, but that one's kind of pricey at $249.) The Sage is a much wider (taller?) knife than the Delica or Native 5 due to having both a leaf-style blade and a bit of a thumb ramp. Still carries just fine for me and doesn't obstruct my pocket at all, though.
If you're interested in moving to a larger overall knife, you really can't go wrong with the Manix 2. I don't own the G-10 version, but the lightweight is definitely a "hall of famer" in my collection and gets tons of pocket time.
And finally, as Deacon mentioned, the Stretch is also a fantastic choice if you want a larger do-it-all knife and don't mind FRN handles or the "alligator like" shape of it. (I like both of those things about it very much, actually!)
Sorry...reading over this, I guess I really didn't narrow much of anything down for you! The good news is, it's pretty hard to find a "bad" knife at the SFO!