Now I understand why you wouldn't want to keep the black one. Otherwise, knowing you have 2, I would have wanted to keep the more aesthetically pleasing one.
And IIRC, of the early FWD VQ35DEs, the 04 i35 is the year that did not have the timing chain guide issues.
Anyway, I thought the A33s were decent looking cars, with the i35 being the prettiest. I usually gauge my vehicles on what I see inside and what I can see from the outside, both at night and during the day. The i35 headlights and tail lights seemed well thought out, and I liked the old school cornering lamps. [In the M45, if you have AFS, your projectors will swivel to the outside when cornering]
The ergonomics of the A33 platform aren't to be beat, and the i35 had way softer seating than the Maximas... and any M45 you get. The BOSE sound system was punchy, but in the M cars it is more refined. In that the subwoofer might not kick as hard, but the acoustics are more well thought of*.
[Another caveat here, since you can get an M with different tiers of sound system - the surround sound/ home theater one ranging down to the base model with only 6 speakers and no subwoofer - not BOSE]
And all i35s came with the rear sunshade, some with front heated seats and even some with all 4 heated + steering wheel, right? Not that you'll really need those in the desert, but again getting a Y50 M with the rear sunshade and thus reclining and heated rear seats is close to impossible. Other than that, very few i35s had navigation. You are very likely to get it in the M. And you're also very likely to get the heated
AND COOLED seats

.
For every other aspect of technology, the M has the i35 beat.
Now, comparing that to a Q is a different story. The Q will ride better and steering will be a little more numb. If you like technology, you'll focus on the M.
Otherwise, I am pretty sure you can get a Q45 for half the price of an 06+ M45.
This one is huge
I know our i35s were oil consumers to various levels. All VK45DEs do too. However, the early iteration of the VK seems to withstand it better. I am yet to hear of a single 03-04 M45 or 02-06 Q45 with a catastrophically damaged engine due to oil neglect. It might be purely a game of numbers, but you are likely to come across engine stories.
We don't fully understand why the difference exists, but it is generally agreed that good upkeep and paying attention to your oil helps ward off the bad spirits.