I consider myself a Medium-core gamer, playing 10-20hrs a week on the industry:
Nintendo: hates hardcore gamers, they want chumps with lots of disposable income to re-buy every game and system they make, mostly for short attention spans and grandparents that want to spoil - that said I've owned every Nintendo console at some point, the staples (Metroid, Zelda, Smash Bros and Mario) are the only ones I ever want to play - the rest is mostly disposable candy crap.
PSP/Vita: no use for these as I work for a living, all my gaming is done at home. I had a PSP when I had loads of downtime a few years ago and it was great as they have many more full-length console quality games than Nintendo (I admit they have some), but not entirely my bag anymore. The Vita has MUCH better graphics than anything Nintendo can offer if that's a priority...
PS3: This is me. Everyone I know that owns a PS3 is an adult with a job and enjoys playing 90% single-player. The PS3 has all of the mainstream games as 360, and MUCH better and more plentiful first party/console specific games like Uncharted, Resistance, Killzone, Gran Turismo (360 has HALO, Forza, and Gears) and free online mutiplayer. The system is AWESOME for multimedia uses as well, offers all VOD streaming services as well as network link to your PC. Easier and less restrictive to upgrade the HDD. some of the old Fat ps3's can play any PS1/PS2 games as well which i only utilized to a small degree... i also prefer the PS3's controller, its smaller and i think lighter...
360: In my experience the 360 is for tweens and adults that aren't adults... (thats it for my trolling...) 360 gets all the same mainstream games as PC/PS3 and they get a lot of DLC before anyone else. I say they have the better internet/multiplayer service (but it costs$) and I definitely envy the avatars. They offer access to all of the VOD services, their console specific games are of high quality but I say not enough variety or quantity therein.
Nintendo: great for kids or if you MUST HAVE Zelda, Metroid, Mario
PSP/Vita: great if you NEED a portable
PS3: for REAL adults, great as mutlimedia hub, if you're a real gamer you owe it to yourself to experience Uncharted and Resistance
360: for tweens/young at heart, if you must have Halo, Gears, Forza
Hope that expands your outlook, again, solely my opinion based on my experience... sorry if I offended anyone, but hey, this is Amurca!