I would argue that you should base your decision between the two words on two specifics: Is your advice unsolicited (i.e., Did somebody ask you for your advice?)? and Will you be giving your advice in written or verbal form?
If your advice is unsolicited, then you should use "should." Most humans do not respond well to somebody commanding them to do something when they did not seek that person's opinion in the first place. That being said, if you think that your advice is significant enough to offer even without someone's asking you for it, use "should" which has a softer connotation than does "must."
If you are writing your advice to someone, again, you should use "should" because you will not have the opportunity to use vocal inflection, facial expressions, or gestures to make yourself clear. However, if you are verbally offering advice on a dire subject (seeking medical care, counseling, obtaining a scholarship, etc.), you can use "must" to stress the importance of your advisee following your advice. With the right facial expression and/or inflection, "must" could actually be more effective than "should."
"Should" fits with a sentence giving advice to a person, as it reflects your opinion, as in "This is what you should do". If you use the word "must", it comes across as a command and is likely not to be taken as advice. Advice is much more effective if given in a neutral tone, such as "If it were me, I would...". But in terms of the grammatically correct usage of these two words, "should" works, "must" - not so much.
If you are merely giving advice, then "should" is the proper word to use, at least in American English. "Should" means that this is the course of action that you think best, but it is not mandatory for the other person to follow your advice.
For example "You should not smoke."
The word "must" indicates that there is no real choice for the other person. So "must" is something that is more of an order than a suggestion. If you are telling someone what they have to do, then "must" is the correct word.
For example "You must not steal."
So I recommend using "should" in this instance since you are advising someone, not ordering them to do something..
The first thing I would say that, definitely you can use these words. But it is wise to consider carefully when you should use "must" and when not while advising somebody.
Usually, we use imperative sentences when we give advice. But, all advice are not mandatory, since it depends on the person's wish sometimes whether s/he would like to follow these. S/he is not obliged to obey taking them as hard and fast rules. Then you better use "should" in stead of "must". Besides, if the person is an honourable one or senior to you, you can give him/her advice using "should", because, it is generally supposed that already he/she is experienced and old enough.
So, considering the situation, you have to choose your words.