With comparable quality interconnects, the balanced XLR should be better than single ended RCA. But as you say, there may be 2 factors acting against this in your system. To get advantage of operating in a balanced system, both the source (preamp) and destination (power amp) should be truly balanced in their operation. Check out the spec of your kit. Secondly, as you suggest, the Mogami microphone cables may not be as good as your RCA cable. In a balanced true system, many claim that a less fancy XLR cable will sound as good as a fancy RCA one and the advantage of balance cable increases greatly with length. You don't need to spend a fortune on XLRs but they need to match the spec for the job in hand. Perhaps look for a pair of sensibly priced XLRs, but probably only worth it if your amp can take advantage of the balanced signal.
+1
In my own humble experience :
My chain = Esoteric balanced dac > kubala sosna xlr cables > MFA passive balanced preamp > cardass golden reference xlr > tube mono power amps.
I tried replacing either of these Xlr cables with a 20 GBP Pro grade mogami xlr. And the drop in SQ was very minimal. And only noticable in 2 ways, a slight increase in back ground noise, and when the track got very busy, I felt the more expensive cables did instrument separation and organisation better.
I replaced these 20 GBP Pro grade xlr cables between the dac and preamp with a expensive hifidelity HT-CR1 magnetic conduction RCA cables. I couldn't make out one from the other.
Then I tried using these 2 pairs of cables, between my preamp to poweramp. And the cheap XLR actually sounded a smidgen blacker than the costlier RCA's, with everything else being the same.
So in my chain, I feel even a cheap XLR beats a expensive RCA as the whole chain is balanced. I've now moved the expensive RCA's to my turn table setup which is not balanced from phono to preamp.