Juvederm is a bit softer and is easier to inject. As far as feeling the product, once it is settled in the skin, in about a week to 10 days, it is unlikely you will feel it from a sensation standpoint or from a palpable touch to the skin. Some gentle smoothing with your fingers following injection may help spread the filler product, but you don't want to do any aggressive massaging or you could misplace the filler.
Smile lines are natural and you don't necessarily want to obliterate them completely. Only an overfill would allow you to actually sense the product. An experienced dermatologist or surgeon will be conservative in your first injections and if after several weeks you felt you wanted more, a touchup could be added.
Scarring is not, in my personal experience, an issue with fillers.
The pain you are experiencing with post surgical scars may not be the scar itself from the incision so much as adhesions -- a type of scar in a sense, but not the same as an externally viewable scar. An adhesion will bind one organ to another during the healing phase and this prevents normal easy movement within the body cavity.
You are not cutting the skin so no scar tissue should form. The only potential for scarring from filler injections is the tiny scars that would accumulate from repeated needle sticks over many treatment sessions.