I've had a few of each and ended up keeping the 90's. I like them because I like the height for things like jungle val, floating plants, large swords, and my 75's got too cramped looking for what I was keeping. I do like the panoramic look of the 75s. I'll take a 180 over a 220 all day...
This is one of those questions where you can ask 5 people and get 5 different answers and none are really wrong. It depends on your overall plans. Iwagumi setups with lots of carpet and little vertically = go 75. Big plants, lots of hardscape = 90 may suit you better
To your questions:
"In terms of being able to keep different fish, does a 90g buy you anything?"
- A little, for things like angels or discus, or larger fish in general. For most fish, not really except you could keep a few more
"Does the extra 4" of height make it more challenging to light for a planted tank?"
- You'll need brighter light to grow carpets (or more light hungry plants at the bottom), which can invite algae up top. May or may not matter depending on your layout plans. If you're not trying to grow demanding plants along the bottom, you prob won't notice much difference
"I'm just wondering if it is worth the extra hassle of cleaning/lighting/etc."
- Lots of variables to consider: cleaning will get your arms 4" wetter which may not matter depending on your height, or desire to use a stool or tools for reach. lighting may or may not matter much depending on your goals.
It's all up to your overall goals. And what you like today, may be different down the road.
92% of all statistics are made up on the spot.