The only operational flow rate tests I've seen for a 2262 came from Harley K, who has two 2262s and tested both at about 330gph/1250lph. I still find that really odd, I expected their design to be a lot more efficient than that. Personally I really couldn't justify 80 watts of electricity an hour for that little flow.
FX5s routinely run at 600+gph/2250+lph, I tested mine at about 2400lph. It's also more energy efficient, 48 watts on 240 volt power, 51 watts on 110 volt power, to run a 3500lph pump, compared to 80 watts to run a 3400lph pump on the 2262.
They hold about the same amount of media, but the majority of an FX5's is sponges. A 2262 holds a lot more 'bio-media', but unless you have more fish than water an FX5 holds more 'bio-media' than you need.
I am not an advocate of overfiltration but for a 300 gallon tank I definitely wouldn't rely on a single 2262 or FX5 to filter it, unless it's very, very lightly stocked, and preferably full of thriving plants.
I'd consider a pair of large canister filters to be necessary. Buy the 2262 and buy an FX5 too. At least then you'll get the best of both.