I think the cut width of either 2 or 2.5 would be nice if the fabric has enough contrast to stand out and be visible against the different fabrics next to it. You can audition so see what pleases your eye. As far as how to make the decision I've been given this guidance in previous projects: The narrower the border, the stronger the contrast needs to be in order for the border to be effective...and...rations of one half the previous width or twice the previous width looks good because the eye sees a balanced proportion. For example, a two inch border against a four inch block looks better than a three inch border against the same block because the relationship is more balanced.