Hi, lets face it they all do the same(meausre smoke)and the ones you will be looking at will all be "approved" or you can not use them, so as far as I can see it boils down to the following;
Not in order

1) price
2) build quality
3) after sales support
4) presentation of printout(front of house)
5) ease of use
6) ability to load other programs(dual use)

Happy hunting!!
Regards Mark.