The D90 is fine but if you can afford the D300s its much the superior camera. End of.
But knowing nothing about what your want it for or your level of interest in photography only you'll know whether you will need or appreciate the additional features, performance and image quality, or whether you'll be happy with the extra size and weight.
It could be that the D90 is all you need, and you can buy an extra lens with the money you'll save by not getting the D300s.
I'm not entirely sure why it's a choice of two Nikons at rather different price points as a first DSLR - unless you've lots of legacy lenses (which has an impact on which to choose) or access to other lenses, you would be well advised to check other manufacturer's offers too to see which best suits you. You might very well still end up with one of these two - they're both very good cameras - but at least you'll know there's nothing more suited to you out there.