Sure, you could try it, but everyone I talk to who has an FX5 and an Eheim to compare says the Eheim works better than the FX5 primarily because the FX5 clogs faster and the flow cuts down. My Eheim is a beast- I haven't cleaned the filter in months, and there's no noticeable flow decrease. It performs absolutely perfect even when clogged & filthy. On the other hand, I've talked to people who have an FX5 set up to do more biological filtration and they all say after just a couple weeks the flow is cut way down. People who have never run an Eheim almost always say the FX5 is way better than people who compare the FX5 to an Eheim, but I don't know how that works. . .
Then again, some people love their FX5s and wouldn't ever run any other filter. It's all a matter of what you like specifically.
In my eyes, an Eheim is extremely durable- you almost never have problems with them mechanically. They're brilliantly simply with the single open space & you can fill it with whatever you want. And they're so hard working it's not even funny. They just run and run and run, no matter what you do to them.
Out of curiosity, what would you be paying for the FX5?