I researched this a good bit back in March and am currently traveling on the SkyTeam Europe Pass (not the RTW) but also recently went back to SkyTeam to explore their Asia Pass for some upcoming travel. In short, with all of the RTW options: It totally depends on where you want to go. Despite all of the Asia-centric airlines in SkyTeam, they couldn't put together an itinerary that would cover the 10 segments I was attempting to include. In Europe, it was easy and a total steal.

OneWorld has the best coverage but seemed to cost the most. Star Alliance is better with Africa and Asia than SkyTeam. I ended up going with the Europe Pass because I've got more in my SkyMile bank than the rest and just booking segment at a time for Asia.

If you call Delta International, ask to speak with the RTW desk and give them your dates/cities, they're good about calling back with a quote for miles/cost.

Boots N All has a quick comparison of the options with mostly up-to-date info: http://www.bootsnall.com/rtw/rtw-ticket-comparison.html