I don't want to sway you away from tinker, its great MC in itself, but transmuter is also excellent. Have you considered all the options?
Tinker:
+50 int and agil max, +10 PoT
Summon and heal pet abilities.
Transmuter:
+50 Int max, +10 Int, +10 PoT +25 Pr and DR
And allows you to make ogre pills, which is a very usefull buff for all classes, and turns you into an ogre (125 Str, 1250 Hp, and 1250 Hp Max). Last I remember they sold fairly well. The other MCA is a rat-form debuff (-125dex/-125agi), which is okay.
Both can be very useful, but if you end up in a raid guild ogre pills for the MT can be very helpful, and the debuff is not bad, though I don't know if it stacks with shaman debuff, don't see why not though. Do whatever you think helps your toon most.