I think your question is better asked as,....... Do you want a D.I. platform AR-15, or a Piston rifle? With that said why limit yourself to just the S&W, (D.I.), and the Ruger, (Piston)? Personally I think there are better versions of both. Your price limit is high enough to let yourself be more diversified. In a D.I. AR-15 rifle a Colt, while over rated, is still considered to be one of the best. In piston weapons the P.O.F. and LWRC rank among the highest rated. I have both 2 Colts, (a 6920 and a 6940 Monolith), and 2, LWRC Piston rifles, (a Paul Howe CSAT Rifle, and a standard M-6 A-2).
I think both the Colt, (I HATE their buyer be damned attitude), and the LWRC, (super customer service and support), can define their platforms performance at the highest levels. One issue you won't see with LWRC Piston rifles are any issues with carrier tilt. LWRC addressed this issue very early on in the design and production of their rifles. Ruger, not so much. Many have shown carrier tilt issues after only 1,500 rounds. Ruger has reportedly addressed this, and later production rifles have had this corrected.
Another thing to consider is that Ruger, while having been in the firearms business for a very long time, is relatively new to AR-15 production. The SR-556 is their first attempt into this market. LWRC has had their hands in the piston AR-15 market for several years, and the brass in the company has decades of combined experience designing and building piston driven AR-15's.
Colt, (again I hate their corporate attitude and arrogance), has been in the D.I. AR-15 business since Stoner invented the damn thing. They do produce an all Mil-Spec gun, even though I think that is more of a selling point than an actual concern for today's civilian AR-15 buyer. I have to admit above all, their guns run damn well, and of course it goes without saying, have been battle proven in several wars.
Bottom line. You are going to be spending a lot of your hard earned money on this weapon. I would say unless you have your heart set on either the Ruger or the S&W for whatever reason, you might be better off expanding your horizions and going with a different, more proven model of either platform. There are many good guns out there, and it is currently a buyers market , especially for AR-15 rifles. Bill T.