Never been to either; have mulled over the possibility of both. Got some questions that might help the brain storming process...
1.) Do you care anything about topside attractions? Such as visiting pyramids, etc...?
2.) If you want to do that, and find Egypt more appealing (e.g.: historical notoriety, those pyramids), is that something you could do on this trip, or that would be better saved till later?
In other words, if you would like to do topside activities in Egypt in addition to the liveaboard, but you only have time to do travel and a liveaboard at this time, and Palau's topside doesn't appeal to you the same way, then doing Palau now and saving the Red Sea for later (when it's easier to take on an extra few days to your trip) might make sense.
Conversely, if you've more time to use now, and that's unlikely in the future, then taking advantage and hitting the Red Sea now might make more sense.
I'm not factoring in optimal seasons for either, just asking if the topside aspect means anything to you.