I had magnetic signs made for my car doors, not noticed any problem with fading but they're not left on 24/7. This is mainly because I did initially leave them on 24/7 for about a week and one of them got stuck in a small area, required careful removal to avoid damaging the magnetic backing too much. This may well have been caused by some dirt or residue on the door, I'm not sure.
I now make sure the area is properly clean before applying, and don't leave them on all the time.
If your car has had any respray work in the areas where the signs are to be placed, it may be more prone to fading depending on the quality of the respray. OEM paint is generally fairly OK, although there are some exceptions, many reds (in particular Ford) can be prone to fading. Either way you may have to T-Cut the car after the vinyl had been removed to reduce/remove any minor fading.
Read window decal.... not got one of these, but both internal and external ones are available, however internal can affect (and be affected by) the heated rear window, whilst external can affect the wiper. Personally I would consider a full rear window perforated wrap, goes outside, covers the whole area, whilst still letting you see out, any competent sign company should be able to advise you on this.
If you get stickers for the rear window, then you're branding the car 24/7, so removable mag signs for the doors are probably not worthwhile, just get vinyl for there too.
