Probably the wrong forum to get an objective opinion, but considering getting my first AR15. I'm not going to be doing any running and gunning, or tricking it out. Just looking for an decent out of the box AR15 for defense and plinking before we're not allowed to get them. I've read about all the differences between the two (twist rate, fwd assist, dust cover, etc.). The MP15 Sport seems to be more readily available and has been in production longer. Not sure what kind of warranty the Rugar AR556 comes with.
There will be some bias since it's a site dedicated to a certain brand but I will say that people here tend to give other brands a fair shake. Asking about another brand won't be a problem here.
Now, if yow come on here bashing the parent brand of the web site or one of it's products without a solid basis as to why, then there is a good chance there will be a problem. An example of what is acceptable would be like the issue some where having with the barrel on the M&P10. If you wish to see what I'm talking about, mosey on over to that sub-forum and take a look.
In any case, from what I've seen, the Ruger should be a fine firearm as well. It depends on what you are looking for in a firearm. Warranty? Well, I can't say how they are and they don't have a specific written warranty. This is what in the back of my 10-22 manual:
WHY NO WARRANTY CARD HAS BEEN
PACKED WITH THIS NEW RUGER® FIREARM
The Magnuson-Moss Act (Public Law 93-637) does not require any
seller or manufacturer of a consumer product to give a written
warranty. It does provide that if a written warranty is given, it must be
designated as “limited” or as “full” and sets minimum standards for a
“full” warranty. Sturm, Ruger & Company, Inc. has elected not to
provide any written warranty, either “limited” or “full”, rather than to
attempt to comply with the provisions of the Magnuson-Moss Act and
the regulations issued there under. There are certain implied warranties
under state law with respect to sales of consumer goods. As the extent
and interpretation of these implied warranties varies from state to
state, you should refer to your state statutes. Sturm, Ruger &
Company wishes to assure its customers of its continued interest in
providing service to owners of Ruger® firearms.
Take it for what it is worth...