Its pretty hard to go wrong with a FMIC since most are pretty good. As for what to get it depends on what you are trying to do. FBO look to a 7” or Helix style. Light mods a 5” stock fit with no cutting is a good plan. I got a 5” ETS because it suited what I was trying to do & has what appears to be very good internal design. Also at the time was on a group buy. Logs show it to work well holding the IAT down in my case.
They all make horsepower claims but I have no idea how valid the claims are. I got mine to manage the IAT in the Carolina summer & any WHP gain is just icing on the cake.
Thing to watch out for, like downpipes, some can tend to be way overpriced.