- Different shape of the 1st stage (outside)
- Different logo on 2nd stage
- Price

- IMHO no. Unless you want to fancy some girls with Black Pearl for
  instance - looks better

IMHO both ATX100 and ATX200 are waste of money. My choice is DS4/TX50
1st stage (DS4) looks a bit old style, but has one bigg advantage: all
LP ports are the same size. In my case, since I have to change
configuration from time to time between doubles and single tank and
because of the valve configuration, I whould have to change hoses also,
because FST 1st stage (used in ATX100) has one 1/2"UNF and others are
3/8" UNF. If you are confident you will never have to change... than FST
may be a little better, because an angled ports give you some advantage
to properly manage the hoses (but not that big difference I think).

I would also recommend TX not ATX version. ATX is smaller, but TX is
more confortable, especially when it lays on the chin. Also air exchaust
is bigger, which may not look so fancy, but at least prevents bubbles to
go straight in front of your mask and disturb your view like in ATX

A good proof that my preferences are not unique is the new XTX series.
Apeks generally seems decided to give up non standard 1/2" UNF ports and
new 1st stages are equipped with all 3/8" UNF. Also new 2nd stages in
XTX version have this big air exchaust like in TX version. I haven't
seen/tested new XTX series yet, but may be it is a choice (and it is
much better to fancy girls as well :D)

BR
Shrek