Well, i guess your choice is extremely easy. You have chosen two so different phones, that your need will decide which phone you need.
P990i is a executive business oriented touchscreen and a pretty large phone. N73 is the youth targeted multimedia phone.
Please clarify your needs, do you want a rocking music and a camera, slick looking phone, or a phone which allows for better email and word proccessing.
N73 shall give you an excellent camera, music as well as the highly popular S60v3 OS which will allow you to increase its functionality a lot using 3rd party application.
P990i will give you touch screen comfort for easy emailing and word processin. Music and camera are not as good as the N73. It has the UIQ3 OS from Sony, which has software support, but not as much as S60.
My suggestion, N73. And about the diff between N73 and N73 ME.
Before a week or so, the difference was the Software and the color. ME came in black color, had a better music player and supported Stereo Widening. But there was no hardware difference, so by changing you product code, you could update the ME software on you normal N73.
But with the latest firmware update, N73 has been given the ME as well the IE features. But ME is also getting an update in a week or so, so what additional features will ME gets is unknown.
But since ME costs only 1 or 2 K more, comes with 2GB card and in a nice looking black color, i would suggest ME.