I would say the GS500 is OK but the Ninja 250R is perfect for a first time rider, its a great introduction to sportbikes, I used to own one, they are defenitely powerful enough for a first time rider (110MPH) and they are a blast to ride, you can play around on them so much.
Listen to what this guy has to say - he ussually rides a much bigger bike, he explians everything you need to know
http://www.youtube.com/watch?v=GxMWbD-GdXw
Also it has great fuel economy