I love conventions. I've been going to them for over ten years. I started at small anime conventions and graduated to DragonCon and GenCon in recent years. When many writers approach me for advice on what they can do to improve their craft, I usually tell them to find a convention with a good literary track, go to the panels and take notes.
Many fantasy and science-fiction conventions have a series of panels (a track) devoted to writing and publishing. You can learn a lot from listening to professionals talk about their craft and better yet, it can be a great chance to meet other writers and network. Many of the writers that come to conventions are happy to talk shop with you and answer any questions you have.
Now, there are also a lot of writing conferences, and I've been to quite a few of those and they are also a great resource for writers at many levels in their careers. I've found the main differences between a convention and a conference is:
Most conferences cost much more than a convention badge would cost you. Conferences do often include meals in the ticket price (which is super nice) and often offer pitch sessions and chances to hear from traditionally published, well-known authors. Many of these conferences are bringing in top people in the field; they have writers who have hit best seller lists, agents, editors and other writers who have a wide range of professional credits in movies, tv, or games.
Generally, conferences are more focused on writing, and writing alone. Conventions may have a writing track, but it's doubtful that's the whole focus of the convention. You'll have to find the panels in between the cosplay, screenings and fandom discussions. Conferences will be focused on just writing, everyone there is for writing. Both of these have pros and con and since many places don't post panel schedules until around a month before the convention it's going to be hard to decide based on the panels.
3. Types of Events
Conventions are a parade of fun. There's always something going on: raves, costume contests, screenings, give aways, panels and more. Conventions offer a great chance to have fun and get some writing advice along the way. Conferences generally have panels and follow a set schedule of limited options. There may be 4-5 panel options for each session time but they are all writing focused. Generally there will be a lunch break in the day and panels end by 8. Many conferences also offer the chance to pitch agents and editors which is an incredible opportunity.
So how do you discover what's the best option for you? There are several questions to ask yourself before you make that decision, so let's break it down.
1. What information am I looking for?
Do I want information about publishing, about overcoming writer's block or about how to write a good opening line? Do I want to know about sci-fi and fantasy markets or am I more interested in romance? The genre you want to write it can help steer you towards one or another.
2. Do I know any writers going?
This may be a strange point to bring up, but if you have a writer you really admire see what conventions they're going to. If they're on panels you can learn from your favorites (and get the chance for a signed book!) while meeting other writers. It's a good way to find events to attend, but if you stumble on a convention or a conference featuring all unfamiliar writers, please don't turn up your nose at it.
3. What is my budget?
Let's be real: going to these things takes money. If you know what your budget is you have a better idea of where/what to look for. An out-of-state multi-day convention might not work if you have about $100 to spare. What might work instead is a small one-day event you can drive to on a day you have off work. Also, consider if you have friends you can carpool/share a room with to help lower costs (and also to help the 'oh god I know no one' anxiety that can spike in unfamiliar places).
4. Where am I in my writing career? Where do I want to be?
This doesn't mean are you a newbie or are you rolling in that sweet published cash (hahahahahhahahahahahahah), instead what I mean is are you just starting out and need to know the basics, or have you published a few stories and are looking for tips to take your career to the next level. If you aren't interested in an agent/editor/traditional publishing path than conventions are likely the better fit for you (though there are some excellent self-published conferences) than a conference would be. Be honest about where you are and where you want to be, let that steer you.
Those are the best ways to break down the big decision of what kind of convention or conference to attend. As a shorter version, my general opinion is that when you're just starting out and interested in genre fiction like fantasy or science-fiction, start at conventions. I say that for several reasons. First, if you're writing in sci-fi and fantasy you're likely a fan so you'll have plenty of other options between writing panels. Secondly, you're more likely to find friends to go with you to share the costs, and finally, it's more affordable.
Just know that conventions and conferences are great places to learn all about writing and to take your career to the next level.