I've done both and loved both.
Which depots/stations are you looking at? Working at Euston station is obviously very different from working at a smaller station up the line, and with the on-board side, there are also differences between different depots.
Other than that, both jobs have their pros and cons - I found that on-board there was more variety, and it could be more fun. On platforms though, because you were always working with the same people, you got to know them better, there was a bigger sense of team work. Platforms can be more challenging in terms of your use of people skills (you get a lot more abuse) which is both a good and bad thing.
Shift patterns vary so much I can't even comment on that without knowing what location you're at, but it can have a big impact.
T&C's are different as well. Onboard is normally Monday to Friday work, with about 1 in 6 weekends that are part of your contract, you can't get out of them. Full time is normally a 41 hour week. Platforms is 35 hours, Sundays are outside, but you can be made to do 1 in 3 of them. If you do, you get paid for them on top of your salary for the rest of the week.