The Megs Glass Cleaner Concentrate dilutes 10:1 with water, so you're getting about 38 LITRES of it for £17 there - should last you a while. It's definitely the best cleaner of the lot, but it doesn't leave any real protection behind, which some can.
If you do want some protection, then look at Duragloss Rain Repel, which is about £7
I have used the Clearkote Vanilla Moose Glaze on my windows the past few times and its great. You can use it on the black trim around the edge too as it doesnt leave white marks. The amount of crap that comes off them is unreal.