I'm not sure ;-; I'm probably gonna choose Mudkip for the following reasons:
*Really wanna use Breloom, which means no Treecko, I like to have diversity (though I did have two Dark Types in Y...)
*Already used Torchic in XY
*When he eventually evolves, Mudkip will destroy Watson, and we wont have a Mega Sceptile by then...
But some reasons I DO want Treecko:
*I love Treecko
*Treecko is boss
*He is Grass
*He has one of the best Megas IMO
*Treecko is still boss
*Treecko was my first and is my favourite starter (but I did play Mystery Dungeon: Blue Rescue Team first, where I had a Cubone...)
EDIT: I'll probably just choose Treecko for AS when I get it, and Mudkip for OR when I eventually get that afterwards (but knowing my luck, I'll get OR first, just like with XY where I bought X first, but couldn't play for two months until I got a 3DS that could play it, whee I got Y and was nice enough to play with my gift first :P)