You're planning on using it, so it's not hypothetical. Therefore using 'would' is wrong, because it expresses hypothetical use (If I fille my suitcase up with books, it WOULD be very heavy) - I'm imagining what the hypothetical situation would be. If you're planning on using your suitcase when you go on business trips or on holiday, that's not hypothetical - it's a definite plan. In this case, you should use 'will'. "In either case, it will be useful."

It "would" be useful for you, because you are hypothesizing about something.

Your question has an error. Should be "I might use it for my vacations" Talking about the business use alone, you would use "will". Talking about the vacation use alone, you would use "would". Talking about them together, I'd probably use "would", but "will" doesn't sound too bad.