One healer is as viable as the other for any content. If you like to continiue playing your summoner go for whm. Spares you the decison to gimp one slightly.

Else if you like AK more go for Sch. If you like WP more go for Whm. But in the end it doesn´t matter - Both can do both with ease. Just more fun with the according class.

Whm got more crybabies in the forums so they will propably get a buff sooner than the Sch.