Battery life I basically see them all as the same.
Screen wise the ole display on the vita is nice and graphic wise yes it is capable of showing off some really nice stuff (far better than the 3DS of course) but the 3D on the 3DS really is great on some games and does enhance some games quite a bit.. not all but some and for me when you have two objects on top of each other and you can clearly see that they are two different objects.. well.
Memory wise.. I give it to the 3DS hands down. the vita you have to buy exspensive spacific memory cards to even use it and you can not save anything to the system or game carts (edit there are limited saves to carts but still require an SD card: article found here: http://www.engadget.com/2011/10/26/sony-limits-ps-vita-game-save-options-memory-card-definitely-no/ ) , the 3DS you can save to the carts, system and SD cards are vvery inexspensive to get. for the Iphone.. well once you get one you have to buy a new one if you want more memory soooo.. yea.
As far as controls... Well both the Vita and 3Ds are the same to me on this. I like the duel analog sticks but the 3DS lower touch screen makes up for this well IMO, yes the Vita has a touch screen but it has the same problem the iPhone has using the touch screen while playig a game covers up half the screen you are trying to look at. I still son't know how the reach around on the vita works so cant really comment on that and the slidepad attachment... no clue.. The iPhone controls.. well again.. I see no point in using it for real games.
Speed... Well this is where the Vita should really shine. The vita should be far faster than iether of the other two.
Launch: well the iPhone basically has everything the other ones had already with little new soo, The 3Ds had a rough start, nintendo played too much to the 3rd part developers and didn't have the stuff together they should have. The vita is shaping up to have a bit more but is also having some problems as well. It will have a few big titles at launch but like the 3Ds most will be not that great and sony has stated that it too will have most of the vita updates several months after launch.. so... we'll leave it at that.
Style: It really depends on what you like.. The 3DS will have more colors in the end and more special editions. but a lot of people like the sleakness of the vita... It just depends on you.
Overall... well overall they all are different and offer different experiances. The 3DS will work with the WiiU, the vita with the PS3 and will also be able to download some ps3 games to it. the 3DS will also be able to transfer some wiiU games though the details are still sketchy. Vita will have 3G, 3Ds will be more group oreinted with much more local player games. Vita will have limited download of older PSP games so you will be able to get some of your UMD games back. the 3DS can play all your DS games. Vita will have flash, 3Ds will have video recording in 3D. oh and the iphone can make phone calls with a 2 year contract if you pay $100+ a month.