Well I think it depends on its use. If you're gonna earn your daily living out of a camera then get the 300D and then with the money earned you'll upgrade the lens system. If you're an amateur and looking for a better way of expressing your artistic talents, then i suggest the 90D. and upgrade on the lenses.
I think its obvious that the more expensive something is, the better specs its got :bsmilie:
I have tried the D300 and the D90 only recently. No doubt the D300 is an amazing camera. This is a fact after using a D200, D100, and D2H. the pro body does feel good with easy access functions etc.
however, you need to know what is your purpose and needs of taking the shots. For $$ ? For a hobby? For school ? I shoot for fun and also do freelance. I shoot with a D50 and used to shoot on a D70.
however, after trying a D90, im really amazed and hands really itching to get a D90 body. Why ? After using Pro bodies before, I found a D300 nothing that amazing. It is a pro body after all, so its kinda "normal" feeling for me. However, a D90 , an "entry" level body is an amazing, totally different kind of feeling type of body.
D50 to D70 not that much difference except d50 better noise and lesser controls.
Comparing D50, 70 and 80 to a D90, the D90 is of a totally different class. I shot a recent "sports" event yesterday and was using a D90. 4.5 frames is good enough compared to my 2.5fps on my d50. Seriously, the noise control was AMAZING !
If you're still undecided, my best advice.. go and try it out yourself. We can tell you all the good stuff, but you wont know how it really feels till you hands on. and Oh, by the way, if you do get a d90, you get spare $$ for lenses or accessories