TV series!
Much more fleshed out story, some things got snuck in that hinted at future events and explained things (the flash back that Shizu had of Agito in their youth, for one example), more characters were involved deeper into the story, and it was set out to be faithful to the manga from the outset (The OVAs were made to cash in on the manga, but out of fear of leaving cool stuff out, Team 5 got intro'd much earlier in the OVAs than in the manga or TV series).
Also, no boobs? The TV series was made with US and European audiences in mind, and I'd bet some offense would be taken at seeing Mizuki naked (the Viz version of the manga had Mizuki redrawn in a bra and panties as well). OVAs can and do get away with a lot more stuff than something intended for TV does. And besides, later in the manga, we get to see Shizu and Val naked, I'd argue there that those looking for fan service didn't miss out on a ton, anyways LOL
Sorry, I just took mild offense to the "no boobs" comment. I can admire the female form, but it's not very fan servicy IMO when Aptom was threatening to kill her and it was more of an attempted rape/murder scene than something I'd consider suitable for titillation, which very little of the manga scenes are aside from Val's ENF moments.