I'd say, in the given though vague example, 720/ultra.
It depends a lot on the game of course, but 'ultra' generally means rendering things like shadows, reflections, more detailed textures and so on. 'Low' means no shadows, no reflections, less detail, less of everything.
Lower definition anti-aliased shadows beat higher definition aliased non-shadows every time.
Plus monitor size/viewing distance play a part. For certain sizes/distances the difference between 720p and 1080p can be relatively small, and for a sub-30 inch monitor it's generally held that there's no benefit if sitting 5-6 feet away or more.