So I followed the advice of my readers, created a night elf warrior, and tested that yes, I could buy heirloom plate items with my Horde characters and mail them cross-faction. Then I leveled that elf to level 10 in 2 hours, and paid for a server transfer, so now the heirlooms are with my level 62 paladin, cross-server *and* cross-faction, for a single 20 bucks payment. And I got rid of the now useless emblems of conquest, valor, and heroism I still had.
Now the heirloom shoulders are just great for my paladin. About the same stats as the ones I was wearing, but now scaling up automatically, and giving +10% bonus to both killing and quest xp. But I'm still not so sure about the chest piece: It does have the same +10% xp bonus, but it has 3 gem slots less than the chest I was wearing. With me having put epic gems into those slots, the heirloom chest has only half the stats of the gemmed chest piece.
Now for soloing the choice is easy: You don't need great stats for solo questing, thus the xp bonus and higher leveling speed is certainly better. But for dungeon groups I'm not so sure. What if the added stats make the difference between a clean clear and a wipe? Wipes tend to lose you a lot of time, especially in PuGs, where some people leave groups after the first wipe and have to be replaced. Wearing the chest with the better stats would make me gain slightly less xp, but certainly improve my performance and success rate.
And then of course the question is whether the fastest leveling possible is automatically the best. I'm opposed to the idea some people have that World of Warcraft starts at level 80. I'm playing alts *because* I enjoy the leveling game too, enjoy visiting content which at the level cap would be too trivial to enjoy. Not to an extreme that I would turn off xp gain or something, but at least enough to make me think whether a 10% xp bonus is worth 30 strength and 45 stamina, which is a lot of stats at level 62.
I'd be interested to hear your thoughts: If there were more items in World of Warcraft which had low stats but each gave some bonus to xp gained, would you try to maximize xp bonus, would you try to maximize stats, or would you go for some compromise?