In my humble opinion, I prefer Kaito over Kiyoteru. I like that Kaito can be used for many types of songs, from cute, relaxing, to creepy and I think his character design allows for some cool fan modules (using the term the Project Diva uses for outfits). Kiyoteru has his nice guy with glasses persona and his glassesless rockin' persona, but I feel like fan designs don't go that crazy with those options. For example, Kaito's hair color can be many shades of blue and he can wear casual clothes to regal clothes and still be recognizable. Kiyoteru is kind of "stuck" with the image of a teacher/part-time rocker. (I am starting to grow to like Kiyoteru more lately, though, and feel pleasantly surprised by what fans create with him, but I think Kaito's still gonna always be a top tier Vocaloid male for me.)
KAITO possesses very pleasant soft voicebanks that Kiyoteru lacks, even when considering his Natural's falsetto. KAITO V1 is also very versatile, largely thanks to the flexibility of the engine itself. However, I only enjoy KAITO V3 Straight when he is tuned exceptionally well; I find his upper register to be whiny, weak, and unpleasant. On the other hand, Kiyoteru is also extremely versatile, and fills the gap in the Vocaloid voicetype lineup, it being strong tenors suitable for rock. This is important to me personally, as I am a fan of rock music and belting. If I wanted to reach for a Japanese baritone voice, I would pick VY2 or Gackpo over KAITO. Although Kiyoteru's V4 voicebanks have weak diction which make notebending more challenging, I am happier with the results I produce with them over KAITO's V3 banks. I have heard some fantastic usages of KAITO V1, but I have never used him myself. As Kiyoteru is one of my favourite Vocaloids, I chose him.
And double wow, because both are no longer tied as.....Kiyoteru is leading the race now!
For me, Kaito is the winner in my heart as he is the first I fell in love with. His voice, his design, and his tragic to a miracle story. Kiyoteru was only when he became a V4 that I started to like him while Kaito was V1 then to his V3. Personal there is something in Kiyoteru's voice that doesn't sit right me sometimes, but to Kaito there isn't really unless it is his English bank and you're doing some unique cover then forgive me Kaito but your ENG bank needs lots of work sometimes
Nonetheless, both are amazing and unique in their own ways. It is also nice to read all your thoughts too guys.