I've used both WHMCS and Blesta extensively but am a big open source advocate. I run Linux servers, use Linux desktops and also wanted an open source billing system. I think Blesta works and looks great and will only get better.
The simple way to think about it - WHMCS is twice the price, but is it twice as good? I would say no - therefore the most economical and financially sensible decision would be to go with Blesta. The biggest thing for me is the community here, it's a fantastic area where anyone and everyone can contribute, where users can actively influence developers decisions. Your ideas are helping mould the softwares future and lay foundations for new features. How can you not be excited by that? How can you not appreciate that? Most companies just don't bother!
I only sell hosting and it's all I'll ever sell, domain management is not important to me in the slightest. Why? Because, no matter what I do I'll never be able to match the prices of the big boys, so why bother when I can focus 100% on my web hosting platform? I sell hosting because I believe I can do it better, there's already lots of amazing registrars out there and if it can save my customers a few quid then I'm happy for them. As long as they continue to host with me then all is good and business goes on.
Just my 2 cents, sorry for the long ramblings