I personally haven't suffered mana problems with pantheon just running mp5 runes (usually on blues since Pantheon lanes against physical majority of the time). If you feel you don't have enough mana, do one or both of these first.
-Get at least 8 in utility for the mana and mana regen masteries
-Start regrowth pendant and go philo stone
Advantage of philo stone is that you get health regen, which is more important than MR since Pantheon will usually fight top (which is ruled by people who can sustain) or bot (as a replacement for the ranged ad carry), both of which is usually physical based damage. Not only that, but philo stone will eventually pay for itself, and its upgrade isn't bad since it gives hp/cdr and a nice MS boost active as well.
Honestly though, Pantheon is strongest at mid-game and falls off late, so I'd rather just go something like brutalizer into BT to take advantage of this. Kinda like leblanc, in that if you don't get ahead early enough, you're going to have problems sooner or later.