^^^^ That decision in entirely up to you.
As I mentioned before an FX5 is overkilling that setup but on the other hand if you get it for a good price then that would be a good pickup and a nice addition, nevertheless I dont see how an FX5 can cost only 70-100 USD more than a CF500. CF 500 should be around 140 uds, you can get the very same canister under the Sun sun brand cheaper, around 110 uds while I've seen FX5 ranging from 280 up to 400+.
Fx5 $309.00
http://www.amazon.com/Fluval-FX5-Ex..._1?ie=UTF8&qid=1372803039&sr=8-1&keywords=fx5
Sunsun HW304 -exactly the same as CF400-. $99.00
http://www.amazon.com/SunSun-HW-304...ie=UTF8&qid=1372803079&sr=1-3&keywords=sunsun
So I dont see it is worth to spend close to 200 bucks more to get an FX5 simply because you wouldnt get anything that you coulndt get from the HW304/CF500, you wouldnt really notice a difference, specially if you keep the two AC110 running.
Cheers.