I have both, If I had to choose one it would be the Glock26. Mag capacity being the number 1 factor. The G26 IMHO is just as easy/ comfortable to carry as the shield. The ability to reload with a higher capacity magazine is another plus.
I shoot my G26 better (Slightly better) than my Shield.
Have a G43 as well, Totally unreliable IMHO, malfunction just about every magazine decent accuracy and fairly easy to conceal, The Shield wins Hands down in the single stack dept, The 43 just cant measure up (Again IMO). Push comes to shove I'd take the G26 over the Shield and the 43. I try to stay away from the 9mm, as a defensive round, I do sometimes carry one. My usually carry pieces are either my G23 with the Surefire XC1 or the Shield 45. Just not a big fan of the under powered 9mm