Well, if both wait until E3, Sony could try to overshadow the Xbox 360 stuff. Do it early and yes, you get your own coverage, but so do MS then. Post-E3 would be able to have a much greater focus on the Xbox 360 if Sony had already shown the PS3 and that big unveiling coverage was already out of the way.
Plus, if Sony only show tech demos at E3, which tend to show graphics you wouldn't see for 3 or 4 years into a console's life (or sometimes never), while MS presumably will show 1st gen titles, to casuals Sony's system's going to seem far better graphically (deservedly or not) as they won't realise (or care) that tech demos are always going to be superior at this stage.
Unless MS show tech demos too of course. Though I don't think that's advisable, it'd just take away the impact of the playable games.