The very important thing to underline here is that they're both going to be extremely different experiences. It's like asking the question "I'm going on vacation to Hawaii soon, should I follow that with a vacation to France?" Sure you can absolutely do both, but they're both going to be extremely different experiences despite the fact that they're both conventions.
Comparing the two, one of them is official and the other is run by fans, this will provide a widely different experience. Bronycon will not have any information about the movie, it will not have new official reveals about the season, it will not show off new Hasbro toys, in a nutshell Bronycon will not have anything new to announce or report in regards to the show. What it will have is fan-produced content, this being unofficial merchandise like plushies and sculptures, artistic work from more fans, Bronypalooza being the big music party stuff like that.
Hascon will not have any unofficial fan works. Don't expect custom plushies or anything like that, there's no rave party, there's not going to be much fan work on display. However, that's where you will find information on the movie, season reveals, brand new toys such Guardians of Harmony to be announced, and products available there before anywhere else, including convention exclusives unavailable anywhere else.
So both conventions have advantages and disadvantages. The underlying difference is Bronycon will provide a fandom experience, while Hascon will provide an experience that is about the future of the brand and "new things".
I hope that clears things up for you. Hascon will also have additional pony guests. I know I for one was heavily disappointed with the lack of any M6 VAs at Bronycon, I'm glad some have been announced for Hascon.