I stayed at Pop. I ended up getting off Space Mountain closer to 12:45 a.m. (as noted in my review, I had the opposite experience of most people who rode Space Mountain and ended up with my longest wait of the night - 15-20 minutes - at the end of the night). I didn't hurry out of the park but was also tired so didn't stick around. There was already a long line at the Pop bus stop. It is right next to Art of Animation's and that line was less crowded, so I decided to get in that line instead (I wasn't comfortable with how packed the Pop line was). However, the buses for Pop were constant - just as one would fill up, another would pull up - and they were also using the longer buses. They filled them as full as possible with people standing. The bus to AoA that I ended up on was a lot less packed, mainly because there were fewer people in line and we had a lot of scooters, wheelchairs, and strollers, which limited the standing room available.
I was pretty tired at the end, so was glad to take a bus. If you're concerned about crowded buses, I would recommend either taking your time to leave MK or comparing the lines for both Pop and AoA and choosing the one you're more comfortable with. If you go to AoA, you can walk over to Pop from there.