A home built in 1991 would have a 3.5 GPF toilet not the 5.0 you are thinking about.
In 1992, the standards went to 1.6 GPF.
The 1.6 will save quite a bit of water on your bill though.
I do notice that the 1-piece designs are a little quieter.
The Toto Ultimate could be a good choice here. Quieter than the Caroma and easier to clean.
If water saving is of more importance, then the dual flush Caroma will do that.
They will wind up using less water.
Either of these toilets will perform well.
I wish I could sell in MN, but the shipping is too much.
For Toto, I do shipping around Seattle, LA and Houston
and can arrange will-call in Brooklyn NY.
I've been sellng the Caroma only in the Seattle area.
Both brands have dealer locators on their web site.
Toto Aquia reviews
Looking inside the tank of the Toto Aquia CST414M dual flush toilet.