It's a good question - you get better battery life from the 2000, and some of the buttons are a little better (the start and select in particular, although they don't get a heap of use in-game), but the screen isn't quite as nice (at least from what I've read). It still looks like a good unit, but if I was hunting for a replacement Vita, I'd probably see if I could find a 1000 for the screen (the 2000s isn't that much of a downgrade, but I'm a big fan of a quality screen).
As for the Vita more generally, there's a decent amount of content on it, although most of the new content is also coming out on PS4, PS3 or PC, but if you're looking for gaming on the go it's a heap better than gaming on tablet/mobile if you prefer buttons and sticks to touch, and the content is much more PC/console-like than the offerings on mobile. I've been playing mine consistently since launch, and with PS+ never run out of content without spending much at all, which is a nice bonus

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For the PS4 connectivity stuff, remote play works pretty well, I've played a game or two of KZ:SF remote play and once I got used to using the touch pad on the back for sprint and grenades I did reasonably, although it is harder lining up shots with the smaller res screen/smaller sticks. It works best for games that don't need a lot of touchpad use on the back, and that don't have overly small UIs (the smaller res screen can make things harder to read). If you regularly lose access to the TV your PS4 is connected to (or were keen to play PS4 games on the balcony in the shade, which is what I use it for a bit, although not so much this time of year), I'd say it'd be a great thing to do, but if you had a mostly dedicated PS4 TV and didn't want to play outside/play in another room while hanging with others, then you wouldn't get so much use out of it. There is a tiny bit of latency as well - nothing huge and I had no trouble adjusting to it, but it is there and worth keeping in mind.
I sound a bit flat about it, but it's mainly because I want to highlight the downsides as they'll usually be the things that make or break a decision. I personally am a huge fan of my Vita, it gets regular play at home when there's the option of a PC and PS4 as well, and is a genuinely great experience. The vast majority of people I know who grabbed one also really enjoy it, but there are a few that it didn't work out for, and I don't want to go "OMG, it's awesome" and then you grab one and not enjoy it

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