Having played all three healers quite a bit (post 65 in all three with an additional 1-65 on another druid, 1-50 on another shaman, and 1-47 on another cleric), I'll tell you shaman are the least impressive of all before you get a bunch of AA's (and relic heal).
As a duo just starting, there is no comparison. Choose druid. Mobility alone for a new character will save you immeasurable amounts of time. Ports and SoW are amazing at low levels especially. Damage shield and regen are wonderful as well.
Cleric will get groups easier usually (unless there's travel involved in which case the druid wins again).
Shaman is a wonderful class against large targets, but his abilities are a bit underwhelming in an xp grind group. Sure stat buffs are nice, but shaman seriously need more group stat buffs. Low levels were god awful trying to keep so many buffs up on everyone who wanted them.
That being said, I'm currently playing a duo of SHD 59 with a SHM 50. SHD is fairly well twinked out though, so the level gap isn't a big deal. If I had my choice though, it would be a cleric or druid. Preferably druid. Travelling fast is just great. Going to XP somewhere, finding it camped and then going elsewhere is just not much of a big deal with the druid. With cleric or shaman it's potentially awful.
Shaman are honestly just way better at being utility and not healers. Yes I said it. Shaman healing is not good unless no other healer is available (though much later they can essentially heal forever... but the heals still suck until Woundbane). Their pet however is far better than the druid or cleric. So you get some nice free dps too. Slows will stretch your healing power, but heals can crit, slows can't. So your slows will never even out the healing power of clerics and druids. If you already have a healer in a group though, shaman slowing and poison DoTs coupled with buffs are really nice to have.
Now if you ask me what I *like* playing in a group... any of the three, so long as people don't expect my shaman to be a good group healer (plus no relic....saaaaad). If I can only pick one as my "main" though. It's cleric, especially if I get to melee. Dat battle spam...