It's quite the hard to pick, Ash's Snivy does get more screen time then Iris's Emolga to show out and show off but, Iris's Emolga has that certain appeal about herself that makes her such a fun and interesting character to watch. In terms of battles, I would defiantly have to with Iris's Emolga, I found her battle against Stephan's Sawk to be vastly more entertaining then every battle Snivy has fought on screen. I just can't get over how strategic and great that strategy was, Emolga didn't stand a chance against Sawk without that strategy since the small flying squirrel was up against a fighting martial artist. However, thanks to Static showing itself and the defensive drop by Sawk's constant Close Combating, Emolga had a fighting chance and finally finished things with a Volt Switch.
I really think Emolga's voice is adorable also XD
I also disagree with the idea that Emolga had a disobedience problem that was sidestepped over, Iris's Emolga has always been established as a lazy Pokemon, but she by no means has no problem battling or helping out in certain situations. She even battled Pignite and Minccino with no problem, and helped Snivy against Simisear. She trained with Swadloon, helped save Axew, got angry enough and protective enough to zap that Swadloon who she thought was bullying the female Axew, and has truly shown some guts as a whole. She even poked fun at Snivy over Leavanny's leaf collar, when it was obvious she didn't like it either, even evading the Vine Whip and taking to the sky(Guess that explains the Grass-Flying disadvantage). I enjoy her interactions with mostly ever Pokemon comes in to contact with, and I just wish she got more battles and screen time and learned Acrobatics already. I defiantly think she's overflowing with personality, and very marketable.
When it comes to Snivy, I like her too. Her personality and interactions are pretty fun for the most part, sometimes I really believe the writers are making Snivy look stronger then she really is. She started off with already awesome moveset and Attract so we can assume her trainer at least trained her well enough, but in many of her battles if Attract fails she often gets wrecked on the spot due to her extremely poor movepool, I really think Georgia's Pawniard deserved to beat her at the Club Battle Tournament, mainly because Pawniard hadn't taken any damage and had an answer to everything Snivy did, it was smart of Ash to use the rocks to hurt Pawniard though. But, Pawniard really shouldn't have been taken out. She's gotten wrecked in mostly every battle where Attract failed, Excadrill, Emolga, for example. When Attract works 100% percent it usually just feels too easy for my taste, her first gym win against Palpitoad for example. She did beat Gothita however, easily. Regardless, I wouldn't mind seeing her evolve in a Servine sooner or later, I just wonder how the writers would showcase her as a Servine, being so much bigger then the other Pokemon and not being a Snivy anymore sort of cuts in her marketability, so I don't think we would see too many more episodes of her playing the big sister role. I just want to see an episode that challenges Snivy as a character, where we truly get to see her struggle on her own in a battle in the same way Oshawott, Emolga, Tepig, and Excadrill have. But, hey Ash's Snivy is too smug to be true and I like her personality, hopefully she starts getting better treatment eventually .
I wouldn't mind her evolving, but I certainly don't think it's happening. Ash's comment about "doing things his way" when Trip remarked Snivy's lack of being a Servine yet, despite her move pool being so advanced made me think she's staying a Snivy. Not to mention the writers have set up a rivalry between her and Trip's Servine and the writers love that whole, "overcoming your evolved form thing."