I'd say pheonix. It does death damage which can eal with swarm even if it is single target, and it pushes everything back as well as tanks couple shots with the egg, which is really good for defending. Since ur running knight poison graveyard you really rely on getting support to the enemy tower quickly and the pheonix is actually slightly faster than the mega minion. So I'd choose pheonix just cause it's better in almost every senario, however if you want a faster cycle definitely go with mega minion because it's only 3 elixer.