Adrien I'm using Upmind and WHMCS interchangeably.
About this, I can say that that Upmind has a better Enhance integration than WHMCS, but both work fine in the basics.
I used Blesta for a while and it works fine but its Enhance integration is third party and I would like it to be in the core just to assure continued support, so I won't be using it in the meantime.
Other solutions I don't know. I don't dare to try HostBill due to the price tag.