I didn't think the D90 was weather sealed while the D300 is (going by memory)?
The D300 has more focus points, more cross-type focus points and shoots faster than the D90. Pretty sure it also has the faster max shutter speed. Don't know if any of that *matters to you*, but there you have it.
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.
D300s would be a better alternative, and since the only difference between them is video and slightly better dynamic range... It would just be a good way to step away from the D90 into a more professional body. The D300 is also not bad, but I feel the D90 slightly out edges the D300 in IQ.
I'm not sure how accurate their list is.. they say D300 shutter lag is 208 ms. I use a precision timer with the D300 for water drops, and for my timing, shutter lag is 54 ms (time from shutter release to firing a flash to capture the picture). An old D70S had shutter lag of 116 ms. I dunno what the 200ms business is?
The D300 is a very nice camera, but it is old technology now, esp regarding high ISO performance and megapixels. If making a change, I'd shoot for a used D7100 or D7200.