the 2 sets of numbers are for offense and defense troops
you will only ever get the 30% of defense troops back - but if you upgrade the infirmary you get more offense troops that are killed when your city is attacked
many feel this is not a valuable upgrade as most keep their offense troops in their acropolis
it does become useful if your acropolis is not maxed and new offense production is left out of the acropolis until you log in again
if you are clearing a siege on your own city - yes you do get your offense troops back for free in the infirmary :)
and it is useful if someone manages to out maneuver you and attack your troops just as they return from an offensive operation
hope this helps
(my infirmary will be one of the last buildings i improve as i believe there are other, better choices to improve first)
it comes down to play-style and personal preference
hope this helps