I fall on both sides of the 15s vs 7s argument. I play two seasons of 15s and several 7s tournaments in the summer every year. In order to stop the schizophrenia, I've just decided to agree to disagree with myself and accept them both for what they are.
They're different. Sevens can be good training for 15s, but can also teach REALLY bad habits. You routinely throw passes in 7s that would get you yanked to the sideline in 15s. Fifteens, I guess, can be good for 7s, as well, but also can be a really bad idea. It doesn't hurt to be physical in 7s (you can totally take the wind out of the other team by hitting them really hard right away). On the other hand, teams that try to play a 15s style generally get burnt out really early. They might get away with it in their first match, but it's hard to keep it up at the pace of 7s. It's also really, really freaking hot in the SE during the 7s season. It's too hot for strategy, you have to just get out and run.