The race of your merc is not going to change anything except your view. A warrior merc will work for you only if you get one of the more expensive ones. Otherwise the merc will taunt the mob off of your pet and get pounded into the ground. The higher tiered mercs can take hits much better than the econo-models.
The downside of having the warrior merc will be your inability to heal him, which may not be an issue if you need a mana break after the fight. If your merc is about to die you have the option of mezzing the mob so long as you have not dotted it.
The cleric you already know about. Low end cleric mercs are slightly more acceptable than their warrior equivelants (sp?), but you will also see a marked improvement of a healer mercs performance based on their tier, aka their cash flow, hehe.
Either way you are going to be missing something you really need to be efficient. I would imagine in the long run you would be best off with the healer merc unless you want to spend all your plat on the high end merc fees. Otherwise you are going to have a tank with no healer...
NOTE: IF YOU RUN OUT OF MONEY WHILE USING A MERC THEY WILL QUIT. They will not suspend themselves, they walk away and you are out of your purchase cost and your merc.
I hope in some way this helps you in your decision. Good hunting and safe journies.