I now have both. As far as the underlying module, the obvious advantage Nailbender offers is multi-mode drop-ins. But since you are asking to compare the 1 Level version, that is a non-issue between the two. By the way, that high voltage module would only be good for multiple cells, especially multiple Li-Ions, so I assume you don't want to run 1x18650, or any single cell format. Also, bear in mind if you opt to drive the LED as hard as it can handle, which is the 1.5 Amps you mention, you won't have a good thermal path in the G2. A M61L would be a better choice than the full-power M61, and Nailbender can provide similar slightly reduced output single level drivers that are safe for extended runs in the G2. For shorter runs, you should be OK, say 10 or 15 minutes max. This is another strong case for the multi-mode in the G2, as you can still get good output on medium, and not worry about heat, and have high for shorter bursts.
The big functional difference is beam. The Malkoff is a real flooder, HUGE hotspot. More like a "hot area". The reflectors that Nailbender uses will give way more throw, although not still in the class of a throw specific light, just a more typical beam with a mix of good throw and a usable spill. The Malkoff is the unusual beam, being as floody as it is. You will hear about a dark ring at the outside of the spill on the Malkoff, and it is there, but in that location, it does not seem to bother anyone, me included. Nailbender's reflectors do not generate a noticeable ring like this. So if you are clearing rooms, doing close work, or walking in the woods where you need to see a broad area in front of you, the Malkoff should work very well. If you ever have need to reach a bit further, the tighter beam of the Nailbender would be the way to go.
As far as the build, the Malkoffs are no doubt more bomb-proof. But if you opt for the thermal epoxy potting for an extra $5 on the Nailbender, I doubt you will ever be able to destroy it, including when mounted to heavy recoiling weapons (i.e. 12 ga. shotgun).
So for me, I like the option of the Malkoff flood, and I have the MD2 light with 2-mode ring so I at least have one lower mode. I like the three mode option on the Nailbender myself, and I also have the advantage of quite a bit more reach, at the cost of close-in versatility.