I played 18/21 and 18/24 (31lv) 1h for a few generations. Now trying out 27/12. Both are good.
Agi shielders are generally a good counter for light armored fast units like archers and ninjas.
Str shielders are ok against anything except maybe cav. They're excellent in formation. Have good survivability if you invest in high armor.
My advice: If u can pull of hitting opponents face like 50 - 60% of the time, and getting 2-1 shotted by almost anyone doesn't bother you- play agi. Otherwise play str.
Or just try both builds on "skip the fun" and see what works for you