I have served in both mechanized and light units and have to say the only differance is in the former you don't have to walk as much and in the latter you don't have to spend endless hours performing maintenance or cleaning vehicles. Of course, that was in my nation's military so perhaps there are key differances in the UK forces I am not aware of.
I've only ever been light role, I have been informed by some mates who're and have been armoured.
In a light role battalion when you're not doing phys, lessons or barrack guards you'll most likely be off playing your xbox (unless the CQMS has some shit jobs that need doing). In armoured units you spend all day every day doing jobs on the wagons, if you have no jobs to do then you'll end up doing the tank park shuffle.
Of course different battalions work in different ways and your Plt Sgt, Plt Comd, CSM and OC will have a massive impact on what you do.