50 is a very sharp lens but on such trips usability is kinda a hassle. I understand when you go up, you will want to capture the entire landscape so 18-55 will work there too. Also, personally i feel it's a little hassle to change lens frequently, agree there will be moments when you will think about doing and you might do it for maybe what 5-10 times but after that you will feel tired. I suppose you will be walking and going over a pony because if so, after walking couple of kms, if you quickly want to shoot, an IS lens is very very handy as might get a little trippy and your body takes a little to stabilize. This has happened to me once, it's not really a deal breaker but hey when you have IS lens use it.
So my vote goes to 18-55 Is and then if there are couple of shots you want to take in night or low light, you might wanna use 50.