I carry the Glock 21 at work and recently bought the glock 43 for back up carry. I also own an M&P and have fired the shield. The triggers on the m&p series of pistols is bad but can be worked out fairly easily. The glock 43 is considerably smaller than the shield and kicks harder too. The trigger on the glock 43 is also more like 6.5 pounds rather than 5.5 pounds on the larger glocks.
Depeneding on what you are looking for, I can not 100% recommend you get rid of the shield. The sights, ergonomics and recoil of the shield are better than the glock 43. The glock is however lighter, smaller and more familiar to other glocks than the shield is. My glock 43 has the meprolight night sights, ghost trigger connector and pearce frame plug. Now that I have done those mods, I love the glock 43 for what it is. You might have a tough choice on your hands.