Heirlooms are generally better than your average everyday leveling green/blue, because they are always going to be 'good' for your level. You very likely wont replace the slots you have heirlooms in until you hit the level they stop working at (so in this case, your weapon slot). As such, you want to pick Heirlooms that will provide the best bonus because whilst you could get lucky and get a bunch of spirit gear whilst you level and have 0 mana problems, you are more than likely not going too, so its better to compensate for that lack of spirit and go for the shield+mace combo.
I don't think Elemental is better than Enhance for leveling. I actually think the opposite, melee generally have a better time when it comes to leveling because of their better movement and the fact they are (generally) able to handle more mobs. Plus standing still and casting really slows you down when you can be fighting mobs and moving to those collect the shiney spots at the same time. However, if you are going to focus your gear on Resto for 5 mans, then it makes sense to level outside of 5 mans as Elemental because you can switch to the staff and use that. If you also have the heirlooms needed for Enhancement and you prefer to play Enhance, then thanks to the new gear system you can level as that instead.
That is the beauty of Heirlooms, and already having both sets. If you do find yourself not needing the mana, you can just change. I've not actually leveled on the beta personally, but I've just been told that mana is sometimes an issue if you are in premade groups who rush through the 5 mans really fast. If you are just going to LFG it, then you likely wont be going fast enough to matter (chaining dungeons often means that you end up doing dungeons the moment you have access to them which in turn means you are likely going to be undergeared for them, so take more damage and thus need more healing, which is where the mana issue comes in).
tl;dr - If you have the Heirlooms for Enhance too, then level outside of Dungeons however you see fit. Whilst in Resto, I would recommend using both heirloom setups, and swap to Staff whenever you feel your mana isn't an issue (level 90-95ish whilst your epics are still useful), and maybe swap into the Mace+Shield as you approach 100, depending on your luck with drops and quest rewards.