If you're trying to make a choice, there's no question, it's a matter of where you are in your career.
For instance, right now I'm in the shifty spot. I love writing conferences, learning new tricks, the whole pitch thing and hearing what people are writing. I've got a lot to be thankful for when it comes to the Willamette Writer's Conference (since it really got the ball rolling toward Happy Hour), so I'm certainly loyal there. But having been published already, it makes more sense (both logically and financially) to reach out to fans, which are few and far between at conferences.
On the other hand, I learn nearly as much by presenting workshops and sitting on panels as I do listening to panelists and participating in workshops.
Conventions are a great place to do promo and have fun and get plastered and gush over your heroes who might just be there promoing and having fun and getting plastered. You'll learn a lot about what the future holds in your genre, because conventions tend to be very genre specific and more importantly, fan-based.
At writing conferences you don't really get any of that, but there you're more likely to make the kind of contacts that could get you published, and you'll likely learn the nuts and bolts publishing information and writing stuff (in a general genre non-specific kind of way).
My tired 2 cents, anyway.
This past weekend was the Pacific Northwest Writer's Conference, an event that I attended as a participant last year and a presenter this year. Attendance has dropped noticeably. I'm not sure what the problem is, but I suspect it's that the panels and workshops were becoming less genre focused. Our workshop on Urban Fantasy Worldbuilding was the only entry in the Sci-fi/Fantasy track. That's a drop from 3 spots last year. Not cool.
You can see why I'm so excited to get my butt to Conestoga. The urban fantasy genre will be on everyone's minds. Even the people who've never heard of it, such is our saturation into that convention. Fangs, Fur and Fey seems to be the largest track, so if you're within driving distance you gotta get over there (and bring some booze, would ya?).
It's also the closest thing we've got to a meeting of the League (three of us in one spot). Maybe next year we can talk Anton and Jeremy into joining the rest of us at RT. We could even throw an official League of Reluctant Adults party.
What are your thoughts on the subject? If you could only budget for one, which would it be, convention or conference?
ETA: I just reread this and realized I must have been on crack when I wrote it. Jesus, so rambling.