An extracting cooker hood captures most steam and cooking smells at source, so there will be less greasy dirt in the air and on top of your units.
An extractor on the wall has to be about three times as powerful as a hood over the hob.
Unlike a window, wind will not blow through an extractor into your kitchen, forcing the fumes to spread around your house, and it creates enough suction, if the kitchen door is closed, to prevent diffusion.
Recirculating hoods are also available. They make an attractive kitchen ornament but serve no useful purpose.
For extractor fans, I mostly look at ducted ones, and VentAxia seem noticeably more reliable than Xpelair.
For a cooker hood, I doubt there is a lot of difference except in price. Elica seem quite good. The "integrated" ones that have a fold out door-covered flap are very expensive, and IME people are reluctant to pull the flap out, so they don't get used much. Some prestige brands are very expensive.
If you are having a wall fan, get a large one and mount it high up near the ceiling. My DM has a 12" one which runs very quietly (small fans are noisier and shift less air) so it can be left on all day at low speed if you want. Electricity usage of a fan is insignificant.
Large wall fans are not widely sold. Always look at the extract rate of a fan or hood, and the noise level in dbs. For example a cheap weedy builders toilet fan might extract a nominal 80 cubic metres an hour, which is very little. A powerful ducted bathroom extractor might shift 240 cu.m/hr. This 12 inch fan can extract more than a thousand and is probably more than you need. I am sure you need at least a six-inch, with a high/low speed controller. Examples
If you are open plan it is especially important to have an extracting hood so it can catch the steam and fumes before they drift round the house. Turn it on before you start cooking, not after.
I would not bother with a humidity sensor. Fried onions or grilled kippers will not start it.
The key thing with any extractor is to start the extractor the same time as you start the hob - so that the vapours start to go out as soon as they start - not just when the air is thick with smoke/ steam / aromas -if you can get into the habit of switching it on every time, you will see the benefit - also if you open a window when you have the extractor on it doesn't actually help ! Either it - not both ! Ceiling extractors seem to be increasingly popular at the moment - just be careful when you want to clean them ! Use a step ladder with someone holding it rather than climbing on the worktop ! Stay safe peeps !!!