1

Either location is good mountain country. If you are new to hill walking, navigation etc, then I would suggest finding a guide or course eg Nav4 in Lake District - see http://www.nav4.co.uk/

You'll want to concentrate on sloping ground, and probably worth practicing using walking poles.

2

I think the Lake District is prettier, although this may not be a concern. I've been many time over the years and like Keswick in particular (although where I used to stay is no longer open so I cant recommend a place). The town however does have an incredible chocolate shop (well you will work the calories off walking so dont hesitate to go in!). You can access walks from the town quite easily, and if you are feeling brave then Scaffel Pike is close enough to take on. If you aren't planning to drive though be aware that you would need to get the train to Carlisle and then bus to Keswick. Hope it helps!

3

Lake district! :)

4

Snowdonia is likely to be less crowded and both would be able to provide you with some decent mountain practice although you won't get any kind of weather or altitude practice. I think the lakes are probably better value but either would be very good experience in their own right as well as for a challenging practice......

5

If you're purely after the experience of a knackering uphill walk, I'd suggest the main path up to Scafell pike.

If you're going before the kids break up, go to whichever is easier/cheaper to get to.
If you're going after the kids break up, go to Snowdon.
If you've got your passport & had your jabs, go to Scotland ;)