For tropheus people say to aim for 7-10x per hour turnover of the tank. So for a 75g you would want 525-750gph turnover. You can achieve that with whichever filters have or you can afford. Your two AC50's move 200gph each for a total of 400gph so you can just add another AC or you can get a canister filter.
There are a lot of good brands out there such as the ones you mentioned. You can go to the review section up at the top of this page to find a good quality filter to buy. The main thing with tropheus is that they need frequent regular water changes. Whatever filters you use doesn't matter as long as it is adequate and performs the denitrification process.
I have 14 duboisi and 12 moliro in a 75g with an oceanic wet/dry reef ready model 3+ sump, a whisper 30-60, and for the time being a sponge filter. The whisper is for mechanical and the wet/dry is for biological. The sponge filter is just providing more bacteria from another tank to help out until the other filters have enough beneficial bacteria. I have roughly 90 gallons of water in the system. Hooray for sumps! <3
Hope that helps. :thumb: