Gimp XB1, just so it can prove hypothetical performance and win DF face off's and keep the warriors happy?
Personally I think the studio should decide.
If the games art style and engine looks suffers bellow 1080p, then choose 1080p
If the games art and engine isn't affected as much by a lower resolution, drop it to 900p if you cant get all the effects and framerate done at 1080p.
I would like if every platform was pushed as much as possible, but I also understand that there's a lot more to the business of making and selling games than that.
DF and the platform warriors. They represent a tiny percentage of the gaming community. Very vocal on forums, but pretty much irrelevant in terms of the gaming market as a whole.
Expecting developers to listen to this noisy minority is ludicrous and destructive. If developers really did pander to this kind of mentality, and spend more and more on pushing each platform to it's limit, do people think the outcome would be:
A) Significantly more sales, offsetting the increased development cost (hint: no)
or
B) Less chance of time and cost budgets being met, no significant increase in revenue, more chance of delays pushing release into an undesirable time-frame, more chance of developer going broke/being cut loose from a publisher. (hint: yes)
The most this action could realistically accomplish is making sales on one platform slightly higher than the other. It's very unlikely to increase overall sales.
There's really very little choice. If you want to stay in business you need to set a baseline (time, budget, image quality, content and features) and stick to it.
'Could squeeze out a few more effects', 'should use 100% of platforms power', and all these other enthusiast wishes are irrelevant when it comes down to a simple man-hours spent vs. revenue equation. Here, in the real world, that equation is pretty much the be-all and end-all.