Does one rule out the other? As in: "If I buy and play Gothic 2, will I never want to play Gothic one again..ever?"
I'll be buying Gothic 2 soon but haven't made it through Gothic 1 yet, so I am just curious. I know Fallout 1 doesn't negate fallout 2 or 3, all are playable and don't "cross" each other out. I just wonder if that is the case for the Gothic series.

Both are equal in terms of pure gaming goodness. Also, the second has some characters that you met in the first who will recognize you from the first. Gothic 2 does a good job of filling you in on the details but wouldn't it be better to have experienced that backstory firsthand??

I played Gothic 2 first, then went back and played Gothic 1. It definitely wasn't as good, but it was still very good. And in some ways it was more impressive since you knew with G1 they were starting from nothing.

I started with 2, loved it, and moved on to 1. Gothic 2 was so much better in many aspects, but that still doesn't stop me from playing Gothic 1 from time to time.
So really, just finish up Gothic 1. You'll understand more of the references in Gothic 2 ;)

Okay another question: can you import your character from Gothic 1 to Gothic 2?

No.
You are the same nameless hero.
As result of the G1 end you are weakened.