Get some prices on the scaff. Main cost is for the transport to get the tube to your place, the manpower to build it then the manpower to dismantle and transport back to base.
I've recently priced scaffolding for all around the house (2 10 metre runs of roof and a 10m gable end)- the quotes varied a bit but the rate for additional weeks was between £25 and £30 per week. I've looked at doing my roof from a tower as well but having done the inner 2 faces (dual ridge) from the comfort of the valley, no way I was going to totter about on a 4' x 6' tower.
If you're slating you have to start at the eaves and work upwards, your ridge dimensions sound like mine but remember, once you've battened and felted (or put your breathable membrane down, whichever) you'll not be able to walk on your ceiling joists. Even applying the felt/membrane is going to be a pain- ascend tower with roll of felt (there's a finite and quite small load capacity on zip towers by the way), step onto ceiling joists, walk all the way to the end of your roof, tack end of felt, hop over rafter, tack felt, hop over rafter, tack felt etc.
Now to fix your battens. Oops- you can't walk on the joists cos there's felt in the way. Yes you can stretch from above the felt but it will be a stretch, you're going to damage the membrane and your setting out will become an extreme chore.
Oh yes, and lightweight modern membrane doesn't half flap about in the wind......
Yes it is possible to do the job from a tower but the risks you are running and the additional obstacles you're facing in order to save a grand or so are quite significant. As someone else said, fall arrest gear is fine but you NEED someone to rescue you within 10 minutes if you've gone over the edge. Fall arrest is like other PPE- a last resort where you can't manage the risk. Fall arrest on a full roof is a bit iffy- if you're at the farthest point from your anchor when you slip there'll probably be enough line to let you hit the ground or something equally solid before the line gets any tension. That'll smart a bit.
And if this is your first roof you'll find it a lot easier working from a stable suitable working platform with handrails and no fall arrest line to trip over