Both of them are going to be very similar in size and very similar in the amount of media they hold. The quality in my opinion is just a slight edge in favor of the Geo, mainly on the connections of the fittings. They are both reverse flow, so chalk one up for each of them. The biggest difference that comes in though is the fact that the Geo uses the larger PanWorld pump, where the PM still uses the Eheim. Don't get me wrong, the Eheim pump is great, but the PanWorld really rocks on that reactor, pushing water through the media much more efficiently and making the reactor itself more efficient.
So with the Geo you would be getting a unit that is a little better built, a lot more efficient, and costs about $130 less than the PM. I would highly recommend going with the Geo units.
Thanks,
Jeremy