Eheim is another brand that is excellent in it's own right. They are initially higher in cost, but they do last for years and years and years...There are a few models that are rated for similar capacity as the FX5.
It's like your basic "Chevy vs. Ford" scenario, just look and see what you prefer.
Both are more than enough for your proposed fish stocking level, but then again, if you spend much time here at MFK, this too will come to pass...
As far as Powerheads, my choice would have to be the Hydor "Koralia"
(
http://www.drsfostersmith.com/product/prod_display.cfm?pcatid=15955 )
They are designed to move water efficiently and they do a great job. Gobs of water movement, without that narrow stream that diseupts the substrate...
You could get by with 2 x #2's , however,
2 x #3's would be even better circulation yet... along with the FX5, that would be a filtration system that will work great for a 240g and the bioload you are thinking about.
Substrate is kinda what you want to look at. My GG could care less what is on the bottom.
Sand, Gravel to Rocks, and everything inbetween has it's pros and cons. To me, barebottom is the easiest to clean but you gotta like the look.
Remember GG's need to pop up for air, so just make sure there is ample room above the waterline for him to poke his mouth out of the water...
Hope this gives you a bit to think about...~Zennz