It's harder to have C+ seat choices as an FO unless you buy it outright. In my experience there are very few times when I get a choice of C+ seat once I get upgraded within the T-24 window; it's usually just assigned and all the other seats in the cabin are shown as taken on the seatmap. And oftentimes it ends up going to the gate for an upgrade and assignment. Not surprisingly, the "better" seats have already been filled by that point, and I generally get upgraded into middle seats or a window seat in a window-less row. (Strange as it may be, I found myself getting the windowless row C+ seat a couple times, leading me to set my upgrade seat preferences for middle and aisle.)