Those are two very different things.
Indeed they are, and I would want it to be understood, especially with a client who showed signs of being a pain in the backside, that while my best efforts would be routinely be exercised on their behalf that no guarantee could be made regarding either one of them.
I suspect that what the OP means here is that he doesn't want some addle-brained nutball holding him "liable" (as opposed to "responsible") for "losses" arising from some wholly misguided financial projections based on entirely false assumptions regarding improved rankings. "Dearie me, we didn't get to No. 1 in Google for everything in ten minutes so now I can't rule the world so I'm going to sue somebody for big bucks out of spite and because I can."
Y'know. Like that.
And I don't blame the OP at all for being cautious. I don't think any contract will help, though. Let's face it, who understands terms in SEO? Who would you call as an expert witness? Let everyone who reads this describe to an imaginary judge right now what a doorway page is. How many different versions did we get? How would a judge determine what was fair and appropriate? Best thing to do with the awkward clients is not get involved in the first place.