Well, as you can see there are varied (and heated) opinions on the subject. Some will profess learning on a dingy and pound that opinion home with their dying breath. It will boil down to personal preference.
I was faced with the same question last year. Should I buy a dingy, learn to sail, then go out and buy the boat I wanted? I read all the heated debate threads.
I decided to just go buy my boat now, learn to sail her, and start enjoying the sport now. My C25 doesn't have all the fancy wind instruments, or even a working knot meter. Only a windex up top and tell tales on the sails. I learned by first reading and studying The Annapolis Book of Seamanship, watching the DVD's, plus a couple other Learn To Sail DVD's....then just went out (in very good conditions only) and did it.
I hoisted my sails, trimmed to the tell tales, watched the windex, and started enjoying my boat and the sport of sailing. Even now, at the end of my first year of sailing and boat ownership, I sail by feel. I feel the wind on my face and cheeks, hair, neck and back. I trim to the look and sound of the sails, and how the boat feels, weather helm on the tiller. I rarely even look at the windex and tell tales anymore. I can hear and feel my trim changes in how the boat behaves and sails. All those fancy instruments would be nice to have, but I don't need them.
Yeah, there have been a few high pucker factor moments.....like when a front came through and I was suddenly grossly overpowered....and I learned first hand what "rounding up" felt like....although she never did as I was able to ease the sheets and spill enough air to keep it from happening, but I sure came close.
Learning to sail is only a small part of the big picture. In the last year I've also learned by doing:
Docking
Anchoring
Mooring
DC Electrical
AC Electrical
Plumbing
Brightwork
Engine Maintenance
Cleaning
Leaks
Marine Head Maintenance
Galley
Gearing up for a weekend aboard
And much more....like the joy of a starry night, gentle rocking while sleeping, waking up with the dawn, fresh coffee in the cockpit, smell of sausage in the galley, hanging out at the marina with other folks, new friends, raft ups....I could go on and on.
None of that would have happened if I went the dingy route...well, the sailing stuff would I guess....but I wanted more than learning to sail....I wanted to "learn the sailing lifestyle".....