One thing that's not immediately apparent with the P180 is that the walls have 3 layers, to reduce noise and resonance. The silicon HDD mounting grommets are much better than the rubber grommets that Antec has used in the past. But suspension system in the P150/Solo is definitely better than grommets at all. With the P180, the PSU is at the bottom so that heat leaving the CPU doesn't heat up the PSU ramping up its fan speeds.
When it gets down to it, you could probably go either case and it won't be a big deal. After all, as others have said it's what you put inside your case that's more important.