Which is the lesser of two evils when dining out,

That is a no-brainer for me: poor service for sure. Unclean surroundings may not only gross me out (depending on the extent of the lack sanitation), but it makes it difficult to judge of the food is safe for human consumption. I will not even eat anywhere that has below an "A" sanitation rating, and a well-ventilated non-smoking section, and so far, that has not been a problem (knock on wood). While service is important, it is not something high on my list of priorities personally. I really don't even demand or ask a lot of the servers. As long as I get my drinks and food in a timely manner, I'm content... and when I don't I realize it isn't necessarily their fault. Luckily every place I have ever been to has always been pretty reputable for the most part.

Both are undesirable.

i would say poor service. because it can be clean with the service poor. i don't care how good the service is, if its not sanitary, then its not safe to eat at.

Personally I'd rather have poor service. Unclean surroundings might just make me lose my appetite. I think service is important but not all bad service is the fault of the workers. Sometimes people are under too much stress or just had a bad day or something. I like to give people the benefit of the doubt when dining out. It's hard to run a restaurant. I'm not excusing poor service, though. Sometimes people are thoughtless and need to step it up.