They're essentially the same thing. Guilty means you agree you did the act that makes you guilty. No contest means you won't argue that you didn't do the act that that makes you guilty. Sometimes a no contest plea makes people feel better about a plea because you aren't actually agreeing you did anything wrong.
Be forewarned, some prosecutors will not accept no contest plea - even though the effect is the same. Furthermore, you can only enter such a plea with the consent of the court - see ORS 135.335(2). So, if you say no contest, also be prepared to say guilty.
Please don't consider this free information to be legal advice. If you want legal advice, you should retain an attorney.
As noted in Mr. Dore's answer, it really depends on the county, the prosecutor, and the judge -- some combination of those sometimes won't accept no contest pleas. A good multi-county DUII lawyer would know. Also, there are times where the difference between the two matters A LOT. Were you in an accident where you weren't at fault? Much better to plead no contest, because that plea does not have "collateral estoppel" effect that bars you from later arguing in a civil matter that you weren't drunk. On the other hand, is there some reason why an angry DA or judge could bar you from diversion (e.g., attitude issues; minor in the car, etc.)? Better to plead guilty and fly under the radar. Finally, in most counties where they let us plead no contest for diversion entry, most practitioners do it -- because it's FASTER. Judges will let the lawyer "stipulate to a factual basis," which is shorthand for, "Judge, I'm a decent lawyer and I've reviewed the facts of this case and it's possible I could legitimately lose it to a jury." This is must (a) faster and (b) less embarrassing than forcing the DA to recite the actual facts of your case into the record for every plea. Life in court is often about "judicial efficiency" -- knowing when and how to use it. On the other hand...in courts where there's no speed-reason to plead no contest or where you're punished for pleading no contest (e.g., in one county, if you plead no contest the judge imposes higher fines than if you plead guilty), it's better to plead guilty if possible. First, in that one county it's cheaper. Second, I've found that folks who plead guilty often have a better chance at making it through Diversion without a revocation -- that is, because they've articulated to a judge, "I had too much to drink and I drove," they seem to take the whole shebang more seriously. Again, your question sounds simple, but it's not -- you need a good DUII lawyer who knows the county, the judges, the prosecutors, and the facts of your case to help you. If you don't have a lawyer on your side, you've got exactly NOBODY on your side in the system. That's a bad place to be.