In terms of bang for the buck, I'd definitely go with the Fluvals. I have run both Fluvals and Eheims, and with the advent of the newer Fluval design (the x04 series) I see no reason to spend the extra $$$ on Eheims. Eheim also encourages the use of its proprietary media, which requires constant replacement in order to be continually effective, and is very expensive. As an additional comment, Magnums are just noisy and small with bad hose fittings and I wouldn't use them again regardless of the price differential.
Since this topic tends to generate a lot of, um, enthusiasm from both sides, let me just say that I have 41 years of fishkeeping experience, and currently run (20) tanks, (4) of them with a total of (6) Eheims and (4) with a total of (5) Fluvals, (and (12) with just Aquaclear HOBs), so I have a bit of field experience on which to base my "opinions".