1 Spirit = ? mp5Follow

1 mp/5 = 3 Intelligence, and 1 Intelligence = 1 Spirit, so 1 mp/5 = 3 Spirit.

Taken from http://www.wowwiki.com/Stat_comparison

Meh, that page really doesn't tell much.
While casting - assuming you have the talents - it takes 10.67 spirit to tick for the same as 1mp5 while casting. It takes 1.6 spirit to tick for the same as 1mp5 outside of casting. So it comes down to where you spend the most time.

Keep in mind weapons can be swapped. If you can, the best setup is to start out with a really high-int staff (like the Aldor revered staff) with an intellect enchant, burn the excess mana that staff gets you, then switch to a high +healing and mp staff for the body (1hand + offhand), and a high spirit (like the one you just got) once you get low and/or get an Innervate. Mostly for long boss fights, of course, where you need to maximize yhow far your mana goes.