When giving advice, "should" is simply a suggestion. You don't need to follow that advice, but it will be good if you do. "Must" has a few uses, but in the context of advice, it's a strong suggestion. You are urging the other person to do this (and they will thank you later). "If you go to London, you should visit the British Museum." (Just an idea. You might like it, but no problem if you decide to do something else.) "If you go to London, you (really) must visit the British Museum!" (Make this a priority. I think you'll really, really enjoy it!)

Well, Must basically means you are required, you have to, you need to. Should, means it would be better if you did, you are supposed to, but it is not required EX: I must take my medicine or I will be sick. I should call my mother cause it is her birthday.