The first picture you posted is of an Encore, the second picture is of a G2 Contender. The Encore is a bit larger and heavier than the G2 Contender, but it will withstand MUCH larger cartridges.
The Contender is capable of withstanding the pressures of cartridges like the 44 Mag, 30-30, 223 Rem, 7x30 Waters, 45-70 etc. All of these cartridges have chamber pressures under 50,000 c.p.u.'s (give or take a little). The Encore will withstand much higher pressures like the 30-06, 300 Winchester Mag, 375 H&H Mag, etc.
The Encore can be chambered in anything that the Contender can handle, but the Contender stops where the Encore picks up. However......
Most of us old "Contender Guys" prefer the Contender as a pistol, although it makes a fine carbine also. The Encore is the way to go if you need a "big" rifle, but it also makes a fine handgun if you need a handgun bigger than say - a 30-30 or similar. I can't shoot much further than 150 yards with a handgun, so the Contender suits me well as a handgun. My brother has been known to shoot 300 yards or better with his 280 Encore rifle. Are you following my drift?
Now as far as differences in the frames - Thompson Center only offers the "new" G2 Contender and the Encore (Encore "ProHunter" is the same basically). If you look around, you can find a good deal on an original Contender. The "Old" Contender is a bit more finicky, but the trigger pulls on most of them are fantastic - that's why they will fetch nearly as much $$ used as the new G2. The barrels from the G2 Contender and "old" Contender will interchange (usually), but the grip and/or stock will not interchange.
Nothing between the Encore and Contender will interchange because the Encore is bigger than the Contender. It comes down to whether you want a large calibers or "smaller" calibers as to whether you want a Contender (new "G2" or "Old-Style") or an Encore (or "ProHunter").
Hope this helps you!