If the Atlas hasn't had the QCGB added (I don't believe that a new nameplate came with the kit), the nameplate indicates it is a change gear lathe. However, my Atlas 3996 (which does have a QCGB) has spent most of its life set to one feed. Unless you expect to do a lot of threading, you won't need to change feed rates too often.
The variable speed is probably nice to have but only so long as it continues to work. A quick internet search turned up so many different prices for the Griz (over a range of almost 2 to 1) that I can't comment on that. The Griz has a slightly larger spindle bore but not enough to make much difference. It has a 4MT spindle nose taper which is probably the least common, and still not large enough to use 5C collets to in my opinion that is either a wash or a disadvantage. In some respects, the Atlas is slightly larger than the Griz.
Odds are that the Atlas will outlive the Griz, and that in 10 years will be easier to find parts for.
The design/size of the 4-way tool holder (turret) on the Griz makes me suspect that you won't be able to mount an AXA QCTP on it. Which is a definite disadvantage.
If we knew a little more about the actual condition of the Atlas, we could perhaps make more comments. But assuming that it was actually or really rebuilt and not just had a flashy coat of paint applied, I would be inclined to go with the Atlas.