Advice? Pre-fab sunroom or build an addition?

I've been considering one of these two alternatives for my back patio for about a year now. I won't be doing the work myself in either case.

I've got a contractor I'm familiar and comfortable with to do a slab and sunroom, as well as re-arrange (re-do) an existing door and several existing windows. But I am unsure whether he is qualified to do a full-blown addition.

At this winter's home show, I asked around for several recommendations, and each time wound up talking to a custom home-builder (showing off much more expensive homes than my own).

Would a custom home-builder capable of very high-end homes be interested in doing a mere addition?

By what factor does anybody guess a home addition is over a pre-fab sunroom?

I have never made up my mind if I am going to live in this house permanently. I like the area but have always been indifferent to my back patio / back yard.

Any body have any thoughts about what I am getting into, and what to do to get the best satisfaction for adding something that's missing about the back of my house?