It depends on the amount of Spirit you have on your gear. I have had a great deal of luck enchanting for spell power and spirit, and splitting gems between spell power and haste. As you progress to the higher level instances, you will find that you get a lot of milage out of having your haste at or near 400 and the healing gear will have haste as a major component of the stats. Spirit is important to keep you up and running in a fight, but haste helps you generate heals at a faster pace. Don't discount your glyphs in all of this. The new rejuvination glyph increases your ticks relative to your haste score. If you are a new 80, its all about spirit and spellpower. Once you start to get more epic gear, keep a close eye on your haste.
In short, spirit helps you generate mana faster and haste helps you throw heals faster. It is a balancing act.
