The WM Ultralite would be more comparable to the Alpinist... 59" girth for Ultra, 61" for Alpinist, 64" for Alpinlite. Also, 14 oz down Ultra, 11.5 oz down Alpinist, 18 oz down Alpinlite.
The Ultralite is 2" narrower in shoulder girth and uses 2.5 ounces more down. Given that it's using more down in less space, it should prove to be a warmer bag. That said, the lightest Alpinist is 21 oz versus 29 oz for the Ultralite.
Crazy that there's a half-pound weight difference between these two. Subtract the 2.5 oz extra down in the Ultra, you're left w/5.5 oz of difference in the shell. From what I can tell, the Quantum Nunatak uses is about the same weight as the WM shell fabric... so the shells should theoretically weigh about the same. I'd guess that the Nunatak eliminates the draft collar at minimum. Perhaps even the draft tube? But that wouldn't make much sense. Whatever... bottom line, there's a definite weight savings in Nunatak; I would guess that the Ultralite would be warmer. I'd be tempted to try the Alpinist with an overfill.