To me, there is no difference on whether the radio button is preselected to either KDE or GNOME, although I prefer GNOME over KDE. Although I don't want to hear/read the complaints by either pro-KDE or pro-GNOME users the will definitely emerge, if such a decision is made.
We already have read the benefits/problems either approach has. To me a pre-selection might also be problematic, because either desktop might not be the perfect fit for every user.
But a Live-CD/Live-DVD environment might be the best approach to solve this issue. It's where part of the success of Ubuntu/Kubuntu comes from. We (KDE- and GNOME-Teams) actually used this to our advantage at Cebit this year. Both booths stood next to each other and in the majority of 'new-to-linux' cases we handed out both Live-CDs to enable them to testdrive each environment at home. The visitors were very pleased.
In a recent 'brainstorm'-like session, an idea was brought up that we might go with a Live-DVD that offers something like 'fast desktop switching'.
It should work with a modified gdm/kdm that offers a 'login' with a one-click GNOME or KDE start. Later you are able to switch from one environment to the other with a special icon, button whatever. If you have made a decision, switch to the environment of your choice and hit the install icon.
=> Users have made a decision on a much more solid ground. => The installation-step in question is gone.
BUT: What about the others, e.g. XFCE, GNUstep...?
Anyway, we should continue to toss ideas around about this issue and see which ones are the most feasible from dev and ux perspective.