XTR 3 is going to have the edge in optical performance over the Mark 5, Leupold has excellent low light performance for a 44mm objective but the XTR III has the 50mm objective for slightly better exit pupil. XTR III has better FOV by far. Burris had some early QC issues they've seemed to have resolved.I’d be interested to hear your thoughts on the XTR3 3-18 vs Mark 5HD 3.6-18. Most discussions focus on their larger magnification models. What’s the optical performance difference in the lower magnification ones? Which has better low light performance?
Here's a couple reviews I did on both scopes:

Rifle Scopes - Burris XTR III 3.3-18x50 mini review and comparison to Nightforce NX8 2.5-20x50
Backstory The Burris XTR II has been the top of the line offering from the Greeley, CO sport optics company for many years now. Initial manufacture of these scopes had some issues optically, but Burris was able to rectify those over time and the latest XTR II is a pretty well refined scope for...
The Bushnell ET LRHSi with G2Hi reticle is really what you're looking for, I have this scope on my SPR and even though it is 4.5-18, doesn't have enormous FOV and is a longer scope, I still love it for this application especially when paired with an RDS