As I understand it, the .XXX TLDs are pushing for exclusive adult use. Therefore if your site has no adult content then you may be in violation of the terms for that TLD.
As Ryan stated, the search engines have no real prefrence to the TLDs, except for in the fact that the users trust them more. Therefore, your CTRs will be higher, which in turn would effect your rankings. When possible .com is the best for most websites.
The .co's have a benefit of being shorter, but they have a huge downside for being too similiar to the .com and that they are not yet widely recognized. Put a .co on the side of service truck and the majority of people are bound to think that you simply forgot the 'm' for the .com.