If I can add my .02...
A little history: my first bike was a yzf600r. Rode THE PISS out of that bike for a year and a half, racking up 20k. commuting, trips to deals gap, twisty rides every weekend...you name it. Next bike was an 03 R1; I've owned that for almost a year now, and have put 13k on the bike...more deals gap, more twisty rides, etc.
Then back in April I purchased an 03 600RR since I decided it was time to get off the street and hit the track. I still own the R1 for the street, and the 600rr is my dedicated track bike.
Here's what I can tell you. It's fun having all that power on tap with the liter bike, but you will inevitably be faster in the corners on a 600. It's easier to throw around, and controlling the throttle in tight corners is simpler since you dont have to worry about breaking the rear loose in lower gears. If you plan on going to the track, the 600 is MUCH easier to throw around, especially if you're riding a tight, technical track. There's plenty of guys that head out to Jennings on liter bikes, and it may be a PITA to pass them on the straights, but it sure is fun shooting pass them in the corners.
I dunno just how fast your buddies are on their liter bikes, but when I had my yzf600r, I had no trouble keeping up with the guys riding liter bikes. Keep your revs up and get a good drive out of the corners...they really won't walk away from you on the straights unless they are running at ridiculously fast speeds. Lower your gearing on the 600 to get a little more pull off the line and you'll be even better off.
Learn to ride the 600 properly, gain some experience, head to the track, THEN after some time decide if you want a liter bike. IMHO, jumping to the liter bike now will only slow the development of your skills on the bike.
oh...as for the "rider weight" issue...I really dont think its a concern. Sure if your 200+ a liter bike will get you moving a bit easier, but if you're a lightweight, it has minimal bearing, if any, on your ability to ride the bike. I'm 135lbs and can throw around the R1 just fine...its all technique. Look at Danny Pedrose on the RC211V...the guy weighs 105#...I dont see him struggling, do you?