Miku here. Been with her longer, so natural bias towards her. Other than that, not many songs on SeeU. Of the Vocaloid 3s, my favorite is probably IA since the base voice was Lia, and some nice originals outside of the Lia song remakes.
Edit: Elaborating on why Miku on the "singing" side. A lot of the composers have gotten used to how to get Miku singing to flow a lot better so it sounds more like actual singing. On the contrary, SeeU is fairly new, so there aren't many songs out, especially considering that IA is dominating the Vocaloid 3 scene since the base voice is Lia, and there are already a lot of good songs that have been put out. In addition, those songs that DO use her haven't really impressed me that much, partially probably because it seems that no one's gotten used to using her yet. Miku, on the contrary has been out for a while, and people are generally used to composing for her. Since I've been following supercell, kz, and tokuP for a quite some time now, it's pretty obvious why my preference is towards Miku. Well, unless something AMAZING comes out voiced by SeeU, but until then, can't really say much for her. (btw this is primarily going off original songs. I tend not to listen too much to volcaloid cover versions of other vocaloid songs)
Visually, probably Miku as well, SeeU's hair is a bit too frizzy for me. Also, my color preference goes towards blue. The black kneesocks probably push her ahead too.