My input: Faced with the same decision
I was looking at the Sachs 125 (Naked bike, very light), the Hyonsung 200 and then the Ninja 250 (Which I didn't consider, because I wanted a really "beginner" bike, and I went to the shop to look at SCOOTERS ! when I found it)
The Ninja 250 is dollar per smile the best value on the market. A 250 is slow enough, the 125, even if it's street legal, will piss you off. Plus, it's an Aprilia and I know you're not getting it for less than the Ninja.
The Ninja 250 is very forgiving and will not only "get you around" for a year, but even if you get a bigger bike, you'll want to hang on to it forever. The Ninja is just so easy, flickable, and good on gas (On top of looking great.. the build quality is way better than the GSRs) it's just so much goddam fucking fun I can't describe it.
And the real reason I'm glad I didn't get something alittle slower is because when I outgrew the Ninja (Like, week 3) I am still happy with what it can do. If I had the Hyosung I would be ACTIVELY looking to upgrade to ANYTHING to get off of it right now. The 250 is great to ride while I save up for the bike of my dreams.
I'm now a big believer in the Ninja 250, even if you're experienced as a rider. It's just such a great, perfected bike it's nuts. I would even look into the 500 as a starter bike. The 500 will keep you happy for quite a while, and allow you to get on the highway at something less than 8000 rpm.
This weekend I also checked out the Suzuki GSR500, And I wasn't impressed. I would ride my 250 anyday over it. It's nice, but the build quality is shotty, the plastics aren't that pretty and the engine is actually built by Hyonsung.
Again, Ninja FTMW.