I'm going to offer a perspective that is probably not popular here with the general consensus. I'm not saying its right or better but just something for you to think about and consider.

First, lets starts with some assumptions/facts:
1. you are a new rider with little to no experience.
2. your desire is to ride a sportbike which implies that you [eventually] want to ride it aggressively.

Now on to my soapbox...

Several here have said that you should get what you want or you'd otherwise be unhappy. I disagree with that. Keep in mind that this is your first bike. If you truly end up loving sportbike riding, then it won't be your last bike. No matter what you get, there will always be something newer and better out there. So there will always be some degree of dissatisfaction.

People have often said that you can start on a 600 as long as you ride responsibly and keep your head. I think that's true 99% of the time. Its that 1% that can kill you. 600s are so smooth (yes, even the older ones) that many are riding high speed without even realizing it. I'm sure we've all experienced looking down at our speedos and saying, "Oh ****! I'm doing 90mph". Now put a new rider in the twisties, cruising along being as responsible as can be and then......BOOM....you in a corner too hot. What do you do???? (there is no right or wrong answer here as there are likely several options) This is where the 600 can be outright deadly and people learn that a 600 isn't as forgiving as once thought.

In these situations, the higher horsepowered bikes would prefer to throw you off and continue on their merry way. But its these situations that you need to learn to suvive on the streets. Things like panic breaking, mid-turn steering adjustments, when and when not to apply the throttle are not easily learned on a 600 (though not impossible). In these situations, the smaller, lighter, less powerfull and more forgiving bike is what will teach you what to do and not do in a safer (and hopefully, less painful) manner. And isn't that the point of your first bike (which brings me to the first assumption/fact)???

Some have said that if you get a small bike as your first bike that you'll be wanting something with more power in a short time. Though I have no evidence or hard data to support this, IMHO, I believe the people who say this are the ones who are only concerned with going fast in a straight line and don't carry the corner speed. A small bike will force you to use all available horsepower and carry the corner speed (which brings me to the second assumption/fact). You'll learn much faster how to ride aggressively and ride a sportbike the way it was designed to. Take it to the track and you'll learn [quite quickly] the advantages to starting on a small bike.

Anyway, this is just a perspective from an avid track rider who pushes his bike more than the average street rider. Ultimately, the choice is yours. Just realize that your choices can have serious consequences. I believe a smaller bike will better prepare you for all the crap that the street will throw at you.