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."