Well, when I was younger I was immediately drawn to Groudon, although my preferences have shifted over to Kyogre...for one thing, it has nicer plushies. Groudon plushies look horribly squashed and deformed. For two, I've always found it more useful. I guess the new abilities change that, but I don't play competitively, so it doesn't make much difference to me.
I prefer the rain to the sun anyway, that probably has something to do with it as well. Groudon brings with it weather I can't stand. But in terms of design I think they're about equal. As version mascots go, I'd say they're better than Reshiram/Zekrom and Xerneas/Yveltal, but not as cool as Dialga/Palkia.