I’ve got a Chromecast, a couple now tv’s and a Android TV box. For bang for buck the Now TV box is great I got a couple of the old ones for £10 and I think it’s great for simply adding iplayer, 4od and ITV player to a TV , I don’t go anywhere near the now TV bits which are just another app. The old one is only 720p which is fine for the kitchens and bedrooms. The new one is 1080p which might be better for a bigger TV. I got the one that came with 2 day sports pass so I didn’t even have to cancel a subscription. There’s a simple hack to install plex on to it which open up loads of possibilities but I’m not bothered for that
As for chromecast, fire stick/tv or Apple TV I’d get the one for the eco-system you already invested in. I’m bit stuck at the moment as I have Amazon prime and google music, one really needs chromecast, the other is best on the fire stick.
I bought the android box to see if could get round it and but to be honest the user experience and the quality of the hardware is not as good as that from the big boys. Toying with getting the fire tv box when it comes out and side loading some streaming apps for US TV shows etc.