I've been using my fluval flex (15g) for over a year, and have mixed reviews to be honest
a) the slats in the integrated filter box are large enough for small fish to get in and out
b) fish can easily jump into the back of the tank and even on top of the filter media
c) as said previously, the filter is too strong for long-finned bettas
d) the filter can be difficult to take apart, although you shouln't really have to (unless a fish gets in)
Most of these are easily modded, but it's annoying to have to do.
a) the heater is easily hidden in the back compartment
b) the stock light grows low light plants pretty well
c) some people really like the look of the bow front
d) it has a remote control light with different colour options, although the lightning simulation seems a bit cruel
If I had to choose between the two though, I'd go with the fluval flex, simply because it's got an extra 4 gallons to stock as you please.
Photos attached below so you can see lighting levels/how I've modded the filter, excuse the crappy set up and photography