HTC is going to add sense overtop of froyo since that's there interface. Because of that, there will be a delay to ensure everything works properly. As for Froyo versions, sure it will probably be different especially if Sprint doesnt release a froyo update for a few months. You can test froyo now on the Evo. Visit the upgrade forum for links to froyo based roms already available.
http://forum.ppcgeeks.com/showthread.php?t=124690