I like them both a lot and would never part with the D90 even though they have the same sensor.
D300 with battery grip is a huge monster semi-professional rig compared to the D90.
I'm no expert on in-camera lens distortion correction but I think the D90 does it but requires manual effort on each individual image.
I use the D90 when I'm in a hurry to grab a camera, the D300 when I want to fool around with a million options.
If the D300 is priced right for its condition grab it. I moved to the D300s / D90 from a D200 and D50. It was a major leap in technology. So far the newer cameras at their crazy Nikon prices have not moved me. My cousin likes his D7200 and I borrowed it for a day and did not feel any need to upgrade.\
There's still lots of comprehensive reviews on both out there on the internet - those by Thom are very wordy but useful.
Try this:
Nikon D90 vs Nikon D300S
And get the 'S' version for much improved handling features and button assignment and placing.