I can't really answer about a sit down machine. I have a Nolting 24" Pro (stand up). Nolting service is prompt and tireless. My dealer is 3hrs away, Nolting Mfg is way far away ......... BUT BUT both are a phone call away or email. I will say Nolting's service is top notch. I really haven't had trouble with my machine (knock on wood).
I belong to a longarm group that meets at a Gammill dealer. There are lot of different machines represented in the group of 60 members. I did try out their Gammill's sit down .... it was nice.