I have owned an Arax 60 and currently own and use an Arax 88. I do not have any first hand experience with Hartblei but when I was trying to learn about the Kiev 60 and which I wanted to spend money about, Arax ended up being my choice.
At the time, there was some discussion of whether or not any given Hartblei camera had actually been built by Hartblei; apparently there were a couple different sellers claiming the Hartblei name. I do not know how that has played out in the decade and a half or so since I read about it.
Not sure if it is still aropund but there was a lively discussion on the Delphi Forums about all things FSU and there was also The Kiev Report. If those are still on line, they might be good resources for info.
I went with Arax for two main reasons: at the time, Arax was the less expensive choice between them and Hartblei. And Gevorg Vartanyan (the owner of Arax) was very willing to answer all my questions before I spent any money, and after he has continued to be will to do so. A minor reason was the ability to order a camera with a custom "leather" covering.
when I ultimately surrendered to my desire for an 88, Arax was again my choice. No issues with my camera or the two film backs I got with it. I've had the camera for about two years now (I think? not quite certain, tbh.) and it's been fine.
Personally, I would probably avoid the 645 option because I think--and this is just my subjective opinion--that the modifications to enable that are mostly adding complexity and may not be the best for longevity and reliability. Also, with the 60, you will be shooting in vertical format which is, for me, not as useful. Like a lot of us, I started with 35mm cameras and have become used to the idea that landscape format is "normal".
When shooting 6x6 I do try to fill the frame but I also will happily crop the final image to whatever seems to work best for the photo and don't really feel limited by the square.
A couple of links to start you off:
http://www.rickdenney.com/The_Kiev_Report/Kiev_report_webpage.htm
and
https://forums.delphiforums.com/kievreport?gid=2087786558
and
https://www.kievaholic.com/
Good luck!