I would go with the pair of 2217. They will more then be enough filtration for your 150g, plus it's always a good idea to have redundant filtration. If the single FX5 were to fail, you may lose all your stock before you can get it fixed, or another filter going.
The redundant filter argument is quite valid, but is it worth spending 2x for an event which may or may not happen? Particulalry, since the FX5 does not appear to be subject to premature breakdown. In the event that the two-filter scenario is compelling (and it does make sense), I would go with the two Rena XP3s. I have had a pair running on a 150 gallon tank continuously for 3 years without even a hint of malfunction.