Both would be absolutely correct, but "in relation to" is more commonly used. The long answer is that they have a subtle difference in regular use. "In relation to" implies a sort of equality between the two objects which are related. "With relation to" implies that the first object is the important one, and the second is just for reference. So you would be more likely to say, "This building is stable with relation to the ground," meaning that the building is unlikely to move or lean. The building is the important part, and the ground is just for reference. It is unlikely that you mean that the ground is not stable. If you said "This building is stable in relation to the ground," you could be saying the same thing, but it is just as likely that you are saying that the ground near the building is not stable. They are of equal importance in the sentence, so you could be drawing attention to either one. I would suggest you don't worry about the subtle difference, however. Both are correct.

in relation to/ related to with relation to both are correct as my dictionary shows meaning - concerning; regarding; with reference to

In relation to sounds a little more natural. I can't remember the last time that I needed to use either of them in normal conversation. It might be used in formal writing. In relation to this question, I am not sure I have a good answer.