Personally, I REALLY want both of them, but I'm a little sad to say that I think, despite being a third-party character, Arle honestly has the better chance. As you said, she isn't as obscure as Lip is these days. Even in their own home country of Japan becoming more and more obscure, whereas Arle gets a new game every 2-3 years or so.
On the contrary. Lip actually has a significant amount of relevance, but I do believe that Nintendo already said at one point that she would never show up, which is why nobody voted for her. When everyone saw the item in Melee for the first time, that is where it begun. And then they just added in a remix of her theme to Brawl and that was the last we heard of Lip.
Also, while Arle has a..."kawaii" design, it's nothing that should be worried about in this day and age of gaming. Technically, I suppose the same could sort of go for Lip, if things like My Little Pony are any indication.
Knowing Nintendo, they will probably use Arle's current design and will probably not have any of the move ideas we are thinking of here. I can also see her final smash being a generic reference to Puyo Puyo solely instead of Madou Mongatari as well, but given the amount of love that Ryu got, which is a mix of both his classic and modern incarnations, I can hope that Arle would have a similar concept. She just has too much potential to not be wasted and I hope we can refute that to Nintendo.
So, looking at this...
Versus...
Lip wins here in terms of an easier 3D conversion. She really only loses with the idea of her moves, which are non-existent. Well Arle, on the other hand, has an array of ideas ahead of her that might never see the light of day beyond her games.
Speaking of her series, and to once again digress significantly this time around because I always like bringing more of this game into the discussion, I always did want them to do it 3D one day. But, I actually have a hard time imagining how it would look without the anime appearance.
When Compile resurrected, they tried to get Puyo Puyo back, but failed. And, they also basically lost Madou Monogatari as well, given Arle is the star of that game; and that all of the monsters in it ALSO appeared in Puyo Puyo. I believe all they have left of that game is Arle's friends that never appeared in the puzzle games.
They were going to try and resurrect Madou Monogatari, as well as return Arle to her roots, but still retain most of the post-Puyo Puyo~n characters and give them a conversion to the prior art style. After that backfired, they made a new game instead for the Vita that is really similar to Madou Monogatari, even receiving a generous amount of praise.
It is the first time that the game was localized, albeit in a reincarnated form. Regardless, it did well and this opens us up to the idea that the actual entries in that series might actually have been very good games, but nobody ever put in the effort to translate them. At least, that I am aware of. Sure wish more people got to play them.
What is even sadder is that most of the original Compile people, except for Kazunari Yonemitsu, returned to this resurrected company. I do believe he is the creator of both Puyo Puyo and Madou Monogatari, so that could perhaps hurt the series a bit if they do somehow get it back. But, given all of the other people came back, they could probably do just as good of a job as they did with him.