I would take the 5.
Switch on - measure - done.
With the 7, you have to pre-select the age EVERY time, otherwise you can't measure.
And the age preselection only changes the display colour/fever limit, nothing else.

Besides the differences in design, the functions are different. Since this comment window is a little too small, here would be the manufacturer's link where all models are listed and can be compared:
https://www.braunhealthcare.com/ch_de...