1. You might have asked them to define how you would make the choice. Find some set of criteria which they could both agree  on, suggested by them. And then you decide.

2. Offer some other "goodie". You say there won't be another exactly like this, but there will be ***something***. A trip; the chance to be first to play with some new tech ...

3. A coin toss. Tell them that you can't decide as you think they are equally good (even though you prefer A; saying this keeps the peace), so you will have to flip a coin (and state ***beforehand*** if it's best of 1, bets of 3, etc)

---------

If you are not an only child, remember something your mother did with you as a sibling. For instance, to divide a chocolate bar, my mother would have one split & one choose. Perhaps one decides an alternative "goodie" as per my point 2) and the other gets to choose whether they want the goodie or to do the presentation.