Are you talking about max level with lots of AAs? Then i don't know. Seems like most the shaman i know love their 2handers these days. If you mean at low levels, like every other class, tank merc will carry you into the 70 range without much trouble. No reason not to keep all your skills up, both offense and defensive.
More info is needed to provide a meaningful response. As mentioned, tank merc is the only choice for all classes until around level ~70 or so. After that, it's either tank or rogue merc for a shaman. Rogue is of course better dps with less AC/HP than a tank. So your choice of which to use varies depending on the relative difficulty of the stuff you are fighting. Choosing 1h or 2h also has variables. It depends what level you are, what type of content you're doing, and what weapon options are available to you. 2h will typically have a better ratio, so that's useful if you're actively meleeing. Shield provides a huge AC bonus which could be beneficial for survival in some situations. It also depends what sympathetic effects you have available. If you post specific weapons you are comparing, you'll get a more specific answer.
