The thing with flexi-face signs is that they are UV printed onto a banner-type material.
UV INK has a micro-texture to it and the banner material is not gloss or completely smooth, and also moves continually in the wind.
To allow for the micro-texture and movement I would definitely want something with a more aggressive tack and minimal shrinkage as possible.
Do not get me wrong, I think the life expectancy of the sticker/job is what will ultimately decide on what you will go for. The following is just a suggestion, there are similar products under different brand names.
I would suggest something like Arlon DPF 7200X High Tack.
Full details and spec can be found here: http://www.Arlon.com
Suppliers:
https://www.williamsmith.co.uk
https://www.spandex.com