What's the difference you ask? Well from my experience, size and what is offered. At conventions more panels are offered than workshops (no always true), workshops provide you with a more hands on experience. Panels are great because you can normally ask questions and are geared towards specific topics.
Now there are some huge writing conferences out there, and since I haven't attended them all I can say is bring a notebook and make a schedule. I've attended smaller ones who put their schedules online so you can plan out your days. This is a great thing to do, take a look at the panels and workshops mark out which ones will be more productive for you and go from there. I love going to the panels at conventions because there's always different views on topics, it's nice to get a wide range of opinions.
Most conference and conventions are geared towards traditional publishing methods, but that doesn't mean they aren't full of information for indie writers. I will be attending one this year geared specifically towards indie-writers and I'm excited to see what they have to offer. I also attend a convention for sci-fi and fantasy in Colorado Springs, COSine, and I love it. There is always notebook pages of information that I come back with. This one is geared towards the readers and authors so it's great.
I'm not going to tell you, like a lot of people, that you have to go to one of these in order for your book to be successful. Because you don't, but if you get the chance (and the money) they are full of great information.
Pros for going:
Networking with others
Over all they can be a great resource for writers. So Google some, and check it out.