I use an FX6 on a 150g and I find it great in terms of reliability and ease of maintenance; it's just not enough mechanical/biological filtration for my needs.
So that's why I bought a second one. Figured it'd be easier in terms of buying media and replacement parts. Plus, I already know the set up so there's no learning curve.
I was looking at the EHEIM 2180 for a while. I loved the idea of more filtration and an in-line heater, but when I looked at the exploded diagram I was a little intimidated because I don't think the Fluval is nearly as complicated or has near so many pieces. Plus, the price. $170 more than the FX6.