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I don't use the XP4, but I would probably opt for two canisters. The flow rate is pretty good there, but most often the flow rates reported are without media so actual flow rate is going to be less. I run a 5 foot 120 gallon with 2 Rena XP3s and it works well for me.
FX5 is better than XP4. You can try that and if not satisfied you can add a HOB or a second canister. It might be enough by itself, depends on stocking.
The fx5 should be plenty for a planted tank, just fill it up with a bunch o media. Also since it is a planted the plants will also help with filtration. You should look into UV sterilizers to get crystal clear water.
I'm a fan of the XP's myself. I probably would go with the XP4 myself, if it were my tank. I'm not a big fan of Fluvals, had one a long time ago and was not impressed. But never had the FX5, so don't know much on it. I've had XP's for over 8 years.