The Wii has already started stealing E3. Sony ironically has an uphill climb. Nintendos known for there booths especially after last year.
I believe that the Wii will have a strong launch in the number of units that get out. I think that Nintendo will have a reasonable amount of units ready for the get go. All pre-orders will be filled with 1 or 2 units extra for each store. Nintendo is relatively good at pegging these things. Then the next month there will be more coming in with possibly a different more complex color to fulfill the remaining demand. At that point Nintendo will have some momentum.
Now one of 2 things will happen. Nintendo will learn from there mistakes and take the ember they made and stoke it to a flame or they'll botch it again and lose the momentum.
Now the PS3 on the other hand is just not going to be able to produce enough units to catch up with the demand and probably won't until 6 months after the initial launch. (Lets face it. God of War 2 is coming to the PS2. That's a system seller right there if it's half as good as the original. Plus its made by Sony itself. Logic would dictate that since you announced it so close to you next gen console that you would leverage it to be a launch title.) Sony can't take as much of a financial hit as it would need to get enough units out initially. If Blu-ray becomes the format of choice then the PS3 will have more going for it after demand can be fulfilled.
Now, assuming that the PS3 and Wii launch at the exact same time, we are 6 months done the road. Wii has more units out because it was able to fulfill the demand and a lower price mark. PS3 finally has enough units out to fulfill demand and will if not already take the leading spots in units sold. This quarter Sony will claim a good launch but a net loss and Nintendo will show a modest profit. The most likely combinations of duel system is still 360 and Wii. The people like webcomic writers will enjoy the Wii but still hang on the 360 because it's rebellious and has a good social system intact. If Nintendo could maintain moment with good solid monthly, or more frequent, game releases on the Wii, any game, they have themselves in a good position to hold dominance. If not they will end up being another Gamecube, a niche player. PS3 is stilled in practice a niche system even though it is still herald as the leader. Adoption is slow due to price and the lack of dedicated PS3 games because the PS2 still has a larger market share and is a known developement plaltform, we saw some of this with the NES/SNES transition.
Coming into E3. Nintendo will work it's E3 magic. MS will offer a more compact sleeker version of the 360 with wireless built in. This is in response to technology prices dropping and the wider availabiliy of Wireless networks because of the Wii, DS, and PSP. MS also announces a portable gaming device based off of its now thoroughly debug Windows Mobile Platform. Learning from Sony MS will use a variant of Flash for games and also include the PDA functionalities because they get them for free. Unlike Nintendo and Sony though MS is releasing a Spec to allow companies like Dell to manafacture these units. This essentially becomes the MS portable and is posed to be somewhere inbetween a PSP and a DS. More powerful then both but with Touchscreen ability. Sony on the other hand releases information on a total redo of the PSP that addresses its fundamental ergonomic problems and will be the focus of the show. That being said Sony, in an attempt to do damage control, will make sure they have a string of there strongest franchises showing up and teasers of the ones that won't be ready for a while, I'm look at you FF:XIII or FF:VII redo, whether the teasers actually become anything but teasers is to be seen. All of these will be coming for the PS3. In fact after a rough start Sony is going to try to give the impression that every game coming out is being developed for the PS3. In actuallity once you stop wearing you Sony Rose Colored Glasses there are still a large number of PS2 games that are coming during the year. Nintendo, not having the same problem since GCN support seemed to dry up a while back, is happily announcing new and innovative things for the Wii. Nintendo will focus on the Wii and DS and by this time how they can get along. GBA, while still receiving games, is left out in the cold. Its too soon to announce its successor but we will probably hear nibbles by the end of the year. During this E3, if Nintendo kept the momentum, we will hear some surprise announcements. One of them being that Square will be re-releasing, again, all of the FF series on the Rev. Either through the virtual console or compilation discs, any of the post snes ones. Sony not liking were this is going. Nintendo talks about how its blue ocean strategy has been taking the world by storm. Also we are told that work has started on the Wii version of Zelda and that its going to touch its roots and be based off of the Original, wooden sword and all. Of course it will be enhanced. As of now Nintendo is seen as 1st place only because Sony didn't have a strong sort. People will still think that Nintendo is in the last place stigma, though facts don't support it. Also still I LOVE HALO 2. Which is ironic because the Wii has a handful of games that are up for reevalution of there ranking to go from M to AO, in particular a very realistic hospital simulation and a mind bender like Eternal Darkness though at this time they are close to release pending the rating change.
I have to go work but I do have more ironically. I hope you Enjoy reading this. It's a little large and covers the industry in the whole but I don't think you can make a good prediction of you don't give what you think the landscape will be.
Edit: Nintendo will also report at E3 its intention to bring some coordination and learning software the to table.
A couple years from now: The Wii is enjoying a niche as a education tool and rehabilitation aid. Who would of thought? Even though games make a good portion of Nintendo's sales. There is more hardware being sold to corporation and government then individuals due to unique training software.