The Tech Pac has the PFS sizing system. Means two things:
1. The torso & shoulder harness can be different sizes
2. Different bladders can be fitted. Standard is the single 65lbs. bladder.
Available are: 24lbs., 35lbs., 44lbs., single bladders, as well as 55lbs. and 85lbs. double bladders. Somebody call that limited ?
Unlike the Tech BC, Ranger BC etc., the Tec Pac has neither integrated weight pockets nor any other pockets standard. Doesn't come with a cummerband either. Twin crotch straps and a webbing buckle are standard, though.
Zeagle has a bunch of different pockets/sizes available, and just about anything (like DiveRite's or Halcyon's weight pockets) made for 2 in. webbing will fit.
It does have, however, a sternum strap and buckles at the soulder straps. Those are available on Zeagle, DiveRite or OMS steel/alu plates if you want them. On Halcyon's DIR set-up they're not used as they're considered possible failure points (all of the above mentioned manufacturers offer DIR continious webbing, too).
The main difference is probably the semi-rigid back vs. solid backplate. That should depend on the dives you do. Mostly doubles, solid plate is really better. But it's a personal preference.