Hi,
For 180g tank. I would recommend at least the Octopus 200 NW or the PS200 (not available yet, probably few weeks). I think those would be your minimum units to run. I think the PS150 would be too small.
The Octopus PRO250 certainly would be an excellent choice and give you a little "oversized" boost. I think same as the RS250 would. We get a PRO250 in a few days to test out, so can't say how it compared to RS250 yet, but from the pics I've seen, it will probably be as good if not better on performance.
From the specs I've seen it will be a higher quality acrylic as well and a more powerful pump.