I installed both in my home as I have 6 bathrooms and by far I like the Sloan Royal flushometer NOT the flushmate.
The great part about a flushometer is the ability to flush one right after another no fill up time.
Less parts to go wrong
Bad part is looks as many people don't care for the commercial look.
Also you need more pressure and larger sized piping then a tank.
Considering they both are supposed to use 1.6GPF the water usage in theory is the same BUT I have found many tank toilets need at least two flushes.
You have to decide if you want a back spud or top spud and round front or elongated type.
Want simple operation that lasts for years and years and is trouble free Flushometer is the way to go Just do not forget to install it as per directions so you can use the vacuum breaker
Tanks sweat, tanks crack ,tank covers crack.
There are a lot of tricks one can play with a fluishometer to get a better flushing action the tank your limited
The tanks have been repaired several times the flushometer are still working the same after being in service for over 20 years with no part replacement or any adjustments and NEVER a stoppage