Both are great, but if you are using a HDT heatsink, or rather a heat sink with a copper base/heatpipe, use Arctic Silver. I believe that the MX-2 is silicon based, so it'll oxidize the copper faster, which you do not want.
Some people also have claimed Arctic Silver lasts very long while the MX-2 doesn't. So if you're a lazy guy, never changing paste for 1+ years, I'd go with AS5. If you change paste regularly, perhaps every 6months, the MX-2 would be a better choice.