There's lots of talk about PS4 Pro, but this thread is one for talking about the different versions of the same system.
So what do you like better? As someone who likes to 'collect' the consoles and keep it as something for my kids, I'm finding less reason to dive in straight away and get the 1st gen hardware even though there is the obvious 'cool and something new' appeal. Long term wise, the second generation hardware is almost always significantly better after the console cycle has finished. It's none more apparent than the new3DS and the PS4 Pro that were just announced. I don't really see any reason someone would want a PS4 launch console 5-10 years later, but I think the Pro would be the definitive version of the machine.
Iterations where 2nd Gen or later were better
PS4 Pro > PS4
PS4 Pro hardware upgrade makes this an obvious choice as the better hardware long term.
PS2 Slim > PS2
The form factor and reduced foot print is a nice upgrade, though maybe not enough to wait 4 years for. Despite removing the HD expansion port which hardly any games used, I still think the PS2 Slim is the better overall model.
PSOne > PS1
I really like the aesthetics of the PSOne over the PS1 but that is a matter of taste.
PSP 3000 > PSP 1000 > PSP 2000
I believe the PSP 3000 fixed the ghosting issues that were in the PSP 2000 so that automatically puts the PSP 2000 last. PSP 3000 is also lighter and thinner so it gets points for that. It probably wasn't worth waiting for the PSP 3000 though.
Xbox One S > Xbox One
Minor touches, but the inclusion of the UHD player in the Xbox One S makes this a better buy.
Xbox 360 Slim > Xbox 360 original
Not much slimmer but the improved reliability over the Xbox 360 original (RROD) makes the latter avoidable at all costs. While the 360 original I believe has backwards compatability with some xbox titles, the slim has the better hardware here and is the safer bet long term.
new3DS > 3DS
Better processor, cute buttons, swappable cover plates, slightly larger screen (19%) and some exclusive games makes the new3DS the version to buy.
DS Lite/DSi > DS
Probably the biggest improvement of all the consoles listed here, the DS Lite looks like the world's best plastic surgeon worked on it and improved it in incredible ways. The thing is perfectly designed from a product marketing standpoint.
Iterations where 2nd Gen or later weren't better
PS3 > PS3 Slim
The US and Japanese PS3 which had hardware backwards compatability with PS2/PS1 games still make the old PS3 worth keeping. In PAL land, it probably is less so, but there's still some decent software PS2 compatability there. Otherwise the form and function of the PS3 slim is still nicer and by no means a console you wouldn't buy if you already had a PS2/PS1.
PSVita > PSVita 2000
This one is debatable and depends how much you like the Vita OLED in the original. The 2000 is probably the overall the better hardware (lighter) minus the LCD screen.