Overall, I say Facebook, and I'll tell you why, and in what exceptional situations Twitter ads are better.
1. Facebook reaches five times as many people. So, unless you are in a business where the type of people who are Twitter users overlaps your best customer set more than most people, you are going to miss 80% of your customers on Twitter.
2. It has at least six times as many targeting criteria. Better chance you'll advertise to the exact audience you want to. Which means less money spent on the wrong people, which means more profits.
3. It has about three times as many different ad formats.
4. It's about five times cheaper. Facebook fans can be had for $0.10 to $1.25, depending on your niche and what you offer. It's hard to get Twitter followers with Twitter ads for less than $1-2.
So, what I allude to in #2 is that there are some types of people who use Twitter more- geeks, marketers, pr people, journalists, for example. So if those are your audience, it's less smart to ignore Twitter and Twitter ads.
Regardless, have a Twitter account and tweet... but where to invest the ad money? Overall it's Google and Facebook 90% of the time.