Emperor 400 for sure!! They work great, last long, and the bio-wheel helps alot. I strongly suggest them.. Also a 50g heavy stocked tank should have atleast 2 filters, doing 10 times per gph at minimum.!
You can never go wrong with more filtration. Go for the aquaclear...very customizable.
I have had mine for over 10 years now.. No problems
Had to replace one 0 ring... That was it.