I am currently at the same cross-roads, and it's very difficult.
But to break down my thoughts,
5D:
-FF advantage over cropped sensor is huge.
-EF mount lenses only, can't use pre-existing EF-S lenses (correct me here gang, since I'm a Nikonian)
-Not great at high ISO's
-No AUTO ISO feature
-IMO, a crappy menu layout
D300
-DX cropped (1.5 or 1.6x factor)
-You have the freedom of using FX and DX lenses, so you can eventually upgrade to a FF Nikon body and use all the lenses you have in your arsenal
-Great high ISO (little noise), but the technology is newer so that is expected
-AUTO ISO and AUTO WB, which for me is a HUGE plus when I am walking around and don't have the time to compose an "on-the-fly" shot; or if I give the camera to someone else to shoot me and my wife in a uber-touristy location
So, I have decided to wait for the 5D MK2 to come out. I thought about switching to Canon (the 1D Mark III with the 1.3x cropped sensor is getting cheap now, and closes the gap on the disadvantages stated above), but $4000 is getting close to D3 territory and I would have to buy really nice Canon glass leaving my lightweight D40 useless (I would still use my D40x when I travel since my wife enjoys it so much and it still takes AMAZING shots despite it's shortcominggs).
So I shall wait for the fall or Olympics, when Canon will release the 5D MK II and Nikon will hopefully release a FF prosumer camera in response (D350 or D400?). I would suggest to wait and rent your equipment from borrowlenses.com until then (great weekly rates on f/2.8 lenses and bodies!)