I have been messing around with my filters for weeks in my 4ft tank.
There are a few things I would think about. First is what fish are going in. I had an eheim 2324 Pro 3 which was really my biological tank, only had biological media in it, I used a spray bar on the surface for my o2 exchange. These have a heater built in, so no heater in the tank and no filter, its an external cannister filter.
My fluval 3+ was the mechanical and it did a great job, but it was massive, however the smaller fish like guppies, danios, red eyes, rummy nose love the underwater current from the output and spend hours swimming against it. The larger fish, basically ones with big fins, hate the fast moving water.
If it were me, I would have an oversized external cannister, as they are easy to maintain and clean etc, and one with built in heater.
If you need more fine filtering, buy a smaller internal filter, like a fluval 2 and your water should be spot on