I went with the AR. Yes, at closer ranges, the shotgun is definitely more versatile. However, it's is limited on range. I also considered capacity and ammo size. I can carry two/three .223 rounds compared to one shotgun round. Also, it's easy to change calibers with an AR. So, you can have an upper and mags for a larger caliber if need be. Yes, it drives the cost up, but the versatility outweighs the cost in my opinion.
Possibly the answer depends on your expected environment?
Keep the shottie, sell me the Marlin (hehe).
After reading all the good points of view I can not decide between the 870 or the AR. But I will buy that Marlin from you.