Out of the two, the Fluvals are *theoretically* less likely to go wrong, as the Fluvals are electronic with no parts subject to wear and tear, where as Visitherms use By-metalic strips that are subject to wear and tear. This said, the E series Fluvals haven't been about very long, and I know one chap that's running a 30 year old Visitherm, that's been going for that entire time floorlessly... Average life on a Visitherm, unless sold on or replaced before it fails is in excess of 15 years in most instances.
As a rule, Fluval filters are noisy, un-reliable, cheap build-quality rubbish IMO. The FX5 broke the mould though, as it's looking good at this point for reliability, noise and some say build-quality, though that's still debatable. Whether or not I'd trust one alone on a 300l tank would depend on what I was stocking with. I wouldn't go there with my Discus, I don't trust Fluval enough for that. With small tetras e.t.c I'd *possibly* give one a whirl. My 300l tank currently runs two Tetratec EX1200's and it's heavily "overstocked". One copes fine biologically on it's own though, but the water needs polishing when working alone
All the best
Rabbut