Right, here goes my 0.02p's worth....
For the record, although I have never owned one, I have used my flatmate's and have to say, I think the E90 is a fantastic device. THe build quality, usabiity is great, although nowadays, it might be considered a little bulky?
The keyboard is HUGE on the E90 and this where you find a big difference on the N97, which is way smaller, and has only 3 rows of keys, whereas the E90 has SIX !!
Perhaps the smaller advantages of the N97 are the 3.5mm standard jack plug, large built in mass memory, TV out capability and of course the slighly better camera, 5mp compared to 3.2mp. These are the about the only things my flatmate craves for his E90. As technology has moved on, the E90 could maybe be manufactured in a much slimmer version, which if it was, I would be very interested in.
THe E90 is still hugely popular out here in Saudi, and still sells brand new ( unlocked ) in quite a few phone shops.
The N97 does of course have the touchscreen, which when it works, works well IMHO, and these days I use Opera 5 Mini Beta version for browsing, which is great to use, and is how other apps for the phone should work. There is a new Firmware version due out any day now for the N97, so if you are going to get one, then wait until this version is released. At the moment, there still are few teething problems - yes, I know, four months after release!
I do think though that anyone going from an E90 keyboard to the N97 keyboard will take some time to adjust. The dialler is on the touchscreen and his its own little pop up button to press.
The N97 Mini is AFAIK, just missing the D-Pad found on its bigger namesake's keyboard.
Otherwise, the N97 can pretty much do the things the E90 can, run Data and Voice calls at the same time, good for browsing ( using Opera ) , good for watching movies, music etc.
Another consideration for you might be the N900? A bit more inline with the E90 I think.
Good luck.