They're both important. It really depends on level of progression, specific fights, and party composition.
Early raiding like kara, you'll want to stack the m/5 for the lasting power; the fights are considerably longer than heroics. As you progress m/5 becomes less important but you'll still want a balance.
For fights that are designed with a lot of movement--aran, gruul's, supremus--you won't be casting as much and can sacrifice m/5 for healing power. Other fights can be heal spams--attumen, kaz'rogal, gorefiend--for which you'll need more m/5 simpily because you're casting more.
Also if you have mustiple resto shamans or a shadow priest in your group, you can sacrifice even more m/5 for healing, relying on them to refill your blue bar.
In short, I find it best to carry a few pieces dedicated more towards m/5 or healing and switching them based on circumstances. Either way, be sure to bring tons of mana pots anyway.