Camaster has a large, supportive user base and user forum. Shop Sabre does not have a user forum that I'm aware of. This can be a big advantage for Camaster.
These two machines are very similar, and the only difference really comes down to support, and who you feel more comfortable with.
VFD manufacturer is irrelevant imo.
Rail size is irrelevant in a machine of this size. 25mm rails are capable of carrying thousands of pounds. Even 15mm rails would be more than adequate.
Since both have the same spindle, I'd expect the one with the pneumatic assist to perform better. Will it make a lot of difference in the way you'll be using it? Probably not much.
My guess is that you'll never need the extra gantry height, unless you maybe want to mount a rotary axis on your table?