I personally enjoy working outdoors however not when you have to stand there, not moving, in cold weather with no way to warm up!! So I'd say Booking Office. Also Booking Office is just an easier job in fact its extremely easy if you end up at a really quiet station with hardly anyone buying tickets.
Gateline at a busy station looks like hard work constantly dealing with drunks, full trainloads of drunks without tickets trying to exit the station on Saturday nights, very high risk of assault, abusive passengers shouting insults, shouting the most really nasty comments imaginable, swearing, awkward passengers, and really difficult passengers. Constant customer enquiries as your the 1st person they come to, constant complaints about anything wrong with the service, and lots of people who arent even customers coming to you from the streets begging you for money or asking you how to get around the town/city you're at. Unbelievable amounts of passengers who can't work out how to put their ticket into the barriers properly.
Memorising the entire timetable (and connections) is required to answer customer enquiries on Gateline as you can be on your own with no info, Booking Office staff have a PC in front of them to find anything out accurately quickly and easily.
Also thats without mentioning the fact that Booking Office also pays another £5-8k more in some TOCs than Gateline.
So go with Gateline if you want a challenge and want to enjoy the good weather, or Booking Office for easier work, higher salary and at least your not standing outside on days or evenings when the weather is poor.