Hello,
I have a FX6 on one of my tanks and I can say that it is a fine filter. To judge which filter would work best for your situation would require more information. Tank size, turtle size and the number of gallons of water would assist in providing you with a better answer. The herpetologists at our zoo recommend a filter capable of 2-3 times the filtration of a similar sized aquarium stocked with fish. Obviously a larger filter within reason would be beneficial. Below is a quick comparison of the two filters....
FX4////////////////// FX6
For tanks up to//// 250 gallons (1000 liters)//// 400 gallons (1500 liters)
Pump output //// 700 Gal/h (2650 L/h)//// 925 Gal/h (3500 L/h)
Flow rate //// 450 Gal/h (1700 L/h)//// 563 Gal/h (2130 L/h)
Filtration capacity ////3.7 gallons (14 L)//// 5.28 gallons (20 L)
Head height (max.)//// 6.9 ft (2.1 m)//// 10.8 ft (3.3 m)
Wattages //// 30 W (120V/60Hz)//// 43 W (120V/60Hz)
....and a link for some extra reading. Please post which filter you decide to purchase.
https://lovefishtank.com/fluval-fx6-canister-filter/Good luck!!!!