A) Kotonoha AI Voice, I get the newest tech on a new program, and if I preorder now I can get a cute acrylic stand, Kotonosync support unsure currently
B) Kotonoha Voiceroid2, I get a confirmed Kotonosync tts, and If I order from AHS, I can get it bundled with Kiritan Cevio AI which I am planning to have shipped very soon
C) Meika Gynoid Talk, I get two distinct talk vbs which are Kotonosync compatible, for two different character voices I love. I have no idea how Gynoid Talk itself fares in comparison to these other programs however
D) Wait for Tsurumaki Maki Cevio AI who I am definitely purchasing in the future anyways...
Uncreepy's post pretty much has all the info you could ever want about them, but I'd like to offer my two cents on your options as well as like... the other person on this site who uses TTS synths.
A. Kotonosync support will most likely come. I think the jump from VOICEROID EX to VOICEROID2 is actually bigger than the jump from VOICEROID2 to A.I.VOICE, since A.I.VOICE visually, at least, looks the same and the only new real feature it has that VOICEROID doesn't (that would actually impact Kotonosync) is voice fusion. But since Kotonosync as it is allows you to utilize VOICEROID2's custom preset option and render with that, I don't think it'll be particularly difficult to update Kotonosync to include it. But that's just from an outside perspective from someone who has absolutely zero experience in actually coding or programming these things, and of course it's up to Suzumofu regardless. But they're a very big Kotonoha fan, so I wouldn't be surprised if A.I.VOICE support for Kotonosync isn't too far off in the future anyway.
B. Kotonoha V2 is more or less the same as Kotonoha A.I.VOICE aside from the new voices. It would be to your monetary benefit if you were to preorder A.I.VOICE this month since the preorder price is cheap, but if you're certain you will get Kiritan soon, Kotonoha V2 is the same price as A.I.VOICE's regular price (after tax and if you're not getting the AHS user discount - if you are, V2 will be cheaper).
C. Like Uncreepy said, Gynoid Talk and VOICEROID are the exact same. The big difference here will be quality between the two companies, and the Meikas.... are not exactly high quality (at least compared to ones from AHS; also Hime's Talk and Vocaloid voices have different tones)? You can find examples on Nico like Uncreepy said if you want to compare, or I can render something for you if you want since I have them all. If you're sure you want to start off with two vocals (Kotonohas vs Meikas) then the Meikas would give you two different ones immediately; the dialect difference between the Kotonohas most likely will not matter to you for a while as a beginner to the language.
D. Also a good option, since Maki will have both Eng and JP banks. There just probably won't be a program made to make her talk banks sing (or maybe there will be, but it won't be anytime soon), but she'll also have Synth V banks to use if you wanted to buy those as well.
I know someone is working on an English patch for Kotonosync. I'd like to not divulge any more details since it's just a side project and needs approval from Suzumofu if it were to be released, but that is a possibility in the future.
I think all of these are strong options, though we don't have any idea of when Maki will release at this time. If you're definitely getting Kiritan soon, I'd just get the Kotonohas with her to at least cut down on shipping, but in the end it's up to you and they're all good choices. Good luck deciding!