Thinkware, along with the blackvue are the most discreet (some low budget options aside), and come with the most features. But as with most things, the law of diminishing returns applies as you go up the price range. If you're into researching try the dash cam forum (do a google search).
Hardwiring along with a voltage protector has the advantage of providing extended parking mode. However, you'd want to be doing a decent amount of daily mileage to get power into the car battery if you go down that road. Some of the mid-range cams like Nextbase will have a small internal battery allowing a short period for car parking mode, but they are cheap affairs and will need replacing as they wear. External 'dash cam' batteries that you might come across are a waste of time and money, as they don't last beyond 18 months use.
So unless extended car parking mode is something you really need, simply use a 12v socket, such as one in the boot.
All F+R cams are pretty easy to install if you put aside a leisurely weekend day to get it right, and are prepared to do a bit of googling for hardwiring if that's the road you go down - and this is from someone who is generally a bit inept at these things.