Sounds like you have been doing your homework. Well, from my experience, stay away from MAC based systems. I type this from a mac.... I love mac machines as a webmaster and a photographer, but reality is that PC's and Linux are the business end of computing. Mac just isn't supported enough by the business world.
Halo is one of our site supporters. They were invited here, and for a reason. Vivo is a long term supported software that will be around 10, 20 years from now and if you can afford them they are your best option by far.
Another i WOULD have supported was NEXTpos but it appears they have been bought out and no longer offer the same type of support they once had. I have been trying to get ahold of Harry (the founder and new CEO), that I did several installs for, and he doesn't have time anymore to even call me back with a few questions my customer's have.
All in all, if you are unsure, consult a reseller that has done installs in your area and talk to the restaurant owners and see if they are happy with their systems. Good luck and let us know what you decide!