To argue Vita's case:
Vita's Internet browser isn't as good as what you'd get on a tablet but does a decent job on most sites and it's browser easily displays pages, multiple windows, and text boxes. A downside is I can't seem to copy text, so unless I'm missing something no copy or paste feature, yet/ever. That included highlighting. You can download photos/mp3/video/music from your browser, I don't have a list of acceptable formats but you can probably find those on the Vita's online manual.
If social media is your thing it has a variety, and an exceptional twitter app I prefer over the website. Facebook is alright, they added a photo upload option but only to your wall, still nice as it is easy to take pictures with.
The camera could be a lot better, but it does a good job as a camera despite low mega-pixels. Which can't be said of many mobile cameras. It also does video.
It has skype so you can call/video call people. Even use it as a phone with skype minutes and a online number, though you might want 3G if you need to be accessible away from wifi. The skype app takes incoming calls no matter what you're doing, even gaming, but you might need the app open in the background but I haven't tested that yet. The mic on it isn't anything special either, but works.
There is a google map app that can get you places but I haven't tried it out much, again more useful with 3G/gps functionality. You can probably chart out directions from a wifi connection and continue to use them while out.
You might think tablets have the edge with apps, but outside of ones found on the PlayStation store, the Vita will be getting PlayStation Suite sometime soon(dev open beta right now). It will provide games and apps similar to what Apple store and Android Market provide. So it will be a diverse multi-tool.
Games. This is where it shines. Unless you need Nintendo IPs, Vita will give you the best gaming experience. Controls are unrivaled in the handheld space. The power of the Vita allows for excellent graphic and gameplay including full online multiplayer that we will see this month with Resistance burning skies's 8 player online matches. Very exciting. Again PS Suite will provide numerous titles but the Vita will have more minis, psn, and psp titles under its belt in time(something like 250+ now?) It will be getting PS1 classics soon and some hope PS2, but there are significant amounts of PS1 titles out there. Vita games themselves will hit their stride later this year and E3 will inform you on them. These games past, present, future, IP, 3rd party will not easily or ever appear on tablet or other handheld devices. Very few devs are seeking out that space, but handheld console from Nintendo and Sony will receive support.
If you have/get a PS3/PS4 you have the advantage of remote play, controller, content manager, cross-play, and free game matching for certain titles. You can use a computer for content management but the PS3 route is probably easier since you won't need to mess with folders, though not hard it's not plug and play like with the PS3.
Battery life is an issue. Reducing brightness/volume, making sure unneeded apps are closed, powering off when not in use, remembering to charge(battery life won't be effected by charging before the battery is low), and not using the livetweet apps backdoor trick to access the browser while running a game in the background will all help keep your battery going all day. Depends on how often you use it, but if your tough on it only expect 3 hours.
Durability and mobility. The thing isn't going to break on you easy, but you need to take care of it so some sort of case is the way to go. The front screen is pretty scratch resistant but the back touch isn't, especially since you put it down on it's back. You can get a larger case or a sleeve if you want to still be able to pocket the thing. I don't suppose this is much different than a tablet except tablets have folding cases while the Vita only has sleeves and zip ups so far.
The UI/OS is pretty good, easy to use and customize. You might find kinks early on until more firmware updates happen. Errors are still few, like lock ups and touch screen sensitivity issues, but are easily fixed by a hard reset. Updates will improve any issues, but you can get insurance if you are worried, though physically breaking it is more likely.
Price point. While more expensive than a 3DS, cheaper than a tablet. Depending on how much memory you need and if you want 3G, the Vita is cheap and packs a punch with its quad core and 5" OLED screen.
Before the PS3 everyone was nice to me :(
