Dynamic res>>>fluctuating frame rate. Very few of the 1080p, 60fps games on consoles are actually 60fps consistently. Hell, very few of the 30fps games can even manage that!
Consider the following:
1) Halo 5 is a video game. Its primary concern should be gameplay.
2) We live in a sadly imperfect world. Amongst those imperfections is the sad state of today's gaming consoles, which are roughly comparable (performance wise) to an entry level gaming PC. As such, technical compromises are, and will remain, a feature of all current gen games, at least for the foreseeable future.
3) Dropping frames is more disruptive to gameplay than dropping pixels (lowering the resolution). See: Wolfenstein TNO vs, I dunno, Assassin's Creed Unity (or any other of the current gen games that struggle to maintain 30 or 60 frames). Which did you find more conducive to a smooth play experience, Wolfenstein's variable resolution or ACU's miserable frame rate? Considering that a not-insignificant portion of players didn't even notice the former, this one answers itself.
Thus, we are left with the following: Halo should be gameplay focused, compromises must be made to achieve technical excellence, and variable resolution is preferable to variable frame rate with regards to gameplay.
All of the other stuff (such as potential min and max resolutions) is entirely pointless to speculate on, as the majority of graphical polish comes in the very last stretch of design.
Also, 60fps>30fps, for reasons that have been elaborated on OVER AND OVER AND OVER. Seriously, guys, get off that hill.