I've been shoooting HFT for a few years now, these are my observations on scopes used.
Objectives, range from 33mm to 44mm, the smaller the better in terms of the number of targets that will be clear to look at.
Magnification, ranges from 8-12, and relates more to how good your eyes are.
Reticles, range from Xhair to Mildot to Xmas tree, and relates more to how many aim points you want/need.
Zero, ranges from 20 to 45 yards, and is more a reflection of your eyes/aim points.
Fixed mag scopes are common but need reparallaxing to airgun ranges and your eyes.
Adjustable objective scopes allow you to fine tune the scope to see more targets clearly.
Prices range from £150 to £700, generally you should think of a scope being about 1/3 rd or more of the cost of the gun.
Average Spring Gun Shooter