AR-15. I debated between the two back... 3 years or so ago when I eventually purchased the AR. The deciding factors for me was:
A- how easy everything was as far as replacement parts or upgrades. Yes you can find anything online. However I prefer walking into my neighborhood gunships than online shopping. The 4 gun stores near me all carried any number of parts, upgrades, and knowledge about the AR.
B- I also felt their was a weight to be associated with the large numbers of Law Enforcement, and personal ownership of these weapons. To me it seemed that everyone who had a choice picked AR's.
I decided to go with the herd and purchased a Stag Arms M 2T. I love it! No doubt it's a fabulous firearm. I recommend Stag Arms to all my friends looking at buying an AR.
Hope this helps.