The grammatical difference is that ‘I will miss you’ is in the simple future tense, whereas ‘I’ll be missing you’ is in the future continuous. In terms of their meanings, ‘I’ll be missing you’ is more likely to be used if you are talking about a specific point in the future, and ‘I will miss you’ would probably be used to talk about a future fact in general. I hope this helps you a little! :)