its v hard to compare eheim vs. fluval.. you will get a lot of contradicting info....
do you have a certain model of eheim and fluval filter in mind?
for those of us who have seen the FX5 in action, few will dispute how good it really is...
the 2213, 15 and above (2222, 2224) are V good filters, but not in comparison to an FX5.
THe "big-daddy" of filters are:
the eheim pro 3e (2076,2078), and the FX5.
an eheim professional 3e (2076, 2078) is comparable to an FX5 and many would argue that they are far better... (you can link them to your computer to program the filter!).
There are many advantages to having 2 smaller filters instead of 1 big filter - personally I like the idea of having 1 filter to clean instead of 2.
hope this helps.....