The difference between a dinghy and a heavy keelboat, is that everything happens more slowly, but has more force behind it. A heavy keel boat is less sensitive to every little puff and shift that would require instant correction on a dinghy.
Generally, if you learn to sail on a dinghy, it'll help you be a better keelboat sailor.
I started with a 25' boat, and upgraded to 30 feet. The difference in handling and difficulty, was not so great that you should feel the need to do this. I upgraded sooner than I expected, because I wanted a boat with better performance, not because I was worried about handling a 30 footer.
Learn to sail on a dinghy, bum a few rides on keelboats to get used to the difference, then go out and buy your 30 footer. You'll be ok.
Amen, brother. Exactly on target. A couple of sails on any keelboat, and you will be golden. Your steepest learning curve will be manuvering under power and docking; your Sunfish experience here is useless, and your 25 foot experience will not be all that helpful in learning how to dock your 30 footer. The 25 footer will have an outboard auxilliary; manuvering with an outboard is completly different than with a fixed prop. With an outboard, you can move the prop around, which give you big flexibility. Additionally, the momentum of a 30 foot boat is a quantam leap greater than that of a 25 footer. Don't get me wrong; docking practice on a 25 footer will not be wasted, but its just a lot easier to do than on a 30.
Another vote for going right to the 30 footer if that is your ultimate plan.