I'm 18 years old and I got my 1999 GT last year.
Personally if you can afford the gas/insurance, GT all the way.
Hower,as far as an 2005 to a 1999, I personally have driven both. I definatly enjoy my '99 to the next gen. The '05 feels much, much, larger than mine. Definatly slower IMO.
Ultimatly its up to you. But for me and im sure most of the people, here find modding the best part of Mustangs. So a little extra cash left would be a nice start.
Hope I helped, either way if your getting a Mustang. Your good with me.