I'll give you my feedback seeing as I have one of the TVs you've been looking at. I bought a Panasonic CX700B 4k TV but returned it due to some uniformity and backlight issues and replaced it with a Samsung JS9000. There are valid arguments for and against 4k.
The biggest argument against 4k at this moment in time is that there isn't a great amount of content, and while that is true it somewhat depends on whether you've seen the TV series that Netflix and Amazon have in 4k. For example, Netflix have Daredevil, Narcos, Breaking Bad, Better Call Saul, and House of Cards all in 4k. If you've seen all of them already then you're not going to have much available to watch. Otherwise, that's quite a few hours of TV you've got there. Movie availability at 4k is currently pretty poor, with only a few films on Amazon Prime and slightly more on Ultraflix.
Secondly, you have to have a good internet connection if you want to stream 4k content - Netflix recommends about 25Mbps minimum.
Thirdly, cost is still somewhat an issue. Sure, you can get a 4k set for £600 or less, but it probably won't upscale HD content all that well, and that's the majority of what you'll be watching at the moment.
Right those are the main arguments against, now for the arguments for 4k. Firstly, cost. If you're thinking of spending anything over £700 then imo you'd be mad to go for a 1080p TV. You can get a really nice 4k set for anything around or over that price, and why would anyone spend good money on a 1080p set that will be outdated in a year or two?
Secondly, upscaling. A lot of this years 4k TVs upscale HD content really well. Even 720p content looks pretty great on my JS9000, and it's really only SD content that looks a bit naff (though still watchable).
Thirdly, content. Although there isn't a lot right now, Panasonic and Samsung have both announced UHD blu-ray players and they should be available by early next year (Panasonic are releasing theirs in Japan next month). Also, Netflix and Amazon are working hard to improve the content that is available and both of them have more 4k shows and films due later this year.
If you have a reasonable TV now then I would advise that you wait. 4k TV prices will continue to fall and by mid to late 2016 we should have much more content available. By then there will be physical 4k UHD content available, which should look superb, plus all the streaming services will have bolstered their content. On top of that, HDR should be more established and common by then, and that is certainly something to be excited about.
I upgraded now because I had a 7 year old 720p TV and didn't see the point in spending £500-600 on a 1080p TV. The JS9000 that I've bought has HDR support and a certain amount of future proofing, plus it upscales HD content extremely well, Xbox One games look amazing and the TV looks absolutely stunning even when it's switched off - it really is a beautiful TV in every way. Sorry for the wall of text, I hope that helps.