Leopard geckos aren't always that happy to be handled, in my experience. By far the easiest to care for, of those two, & cute to watch.

Bearded dragons, on the other hand, are very personable & like to cuddle...BUT, they are fairly high maintenance to do it right. Their food needs to be fresh &
varied daily, they need higher temperatures to bask, real sunshine or proper lights to simulate sun, larger enclosures...I don't know the age of your "little sister"
but she may not be up to the task of keeping a beardie without a lot of help (& financial backing).