The key word in both eldritch invocation descriptions is “can”.
When you hit a creature with eldritch blast, you can push a creature up to 10 feet away from you in a straight line.
Once on each of your turns when you hit a creature with your eldritch blast, you can move that creature in a straight line 10 feet closer to you.
Since you can choose whether to do so, you can also choose not to use one of them, so there’s no reason both would have to be active, nor that one would take precedence.
This means, if you take both invocations, you can decide whether to move a creature towards you or away from you depending on the situation.
That said, Repelling Blast says “When you hit a creature” and mentions no other restrictions, whereas Grasp of Hadar says “Once on each of your turns when you hit a creature”, so at higher levels (when you get more beams via eldritch blast) you can move a creature up to 40 feet away, but only ever 10 feet closer (see this question, which tackles the Repelling Blast case in more depth).
Finally, there’s the possibility of the warlock wanting to use both forced movement abilities at the same time, moving the target 10 feet in one direction, then 10 feet back again. The following is taken from lightcat’s answer:
V2Blast points out that this is in line with the optional rule on Simultaneous Effects in Xanathar’s Guide to Everything (XGE 77):
If two or more things happen at the same time on a character or monster’s turn, the person at the game table – whether player or DM – who controls that creature decides the order in which those things happen.