I'm not real knowledgeable on the AR-15's,but a lady friend of mine bought the Ruger last year,and we both love shooting it. Never had any problems with it! But it's the only AR I ever shot,so I really don't have anything to compare it to.
If I were you, I'd pass on the Ruger...they use proprietary parts, so you aren't going to be able to replace some of the parts down the road. If you have no plans to swapping out any parts, then I'd consider it...but I've never owned an AR15 that I didn't modify at some point. I've owned over two dozen ARs.
Take a look at the S&W MP Sport I & II...they are inexpensive and will allow you to swap parts.
It's plastic delta ring. Unless you are getting the rifle for a steal, I'd recommend getting something else. There are just too many options for a sub $600 AR15 rifle to buy something that isn't to the same specs as other AR15s.
Don't consider Radical Arms either as they are producing some junk.
So plastic delta ring...some reported issues with adding handguards due to the FSB. The BCG is a semi.
Doesn't sound like a great deal to me. Get a rifle kit from PSA...spend 15 minutes to learn how to put the lower together.
I'm assuming your talking about the Ruger AR 556 DI system and not piston driven. I own a Colt, a Stag, 2 PSA's, and a Ruger AR 556. If your just looking to get into the AR thing without spending a lot of money the Ruger is a great choice. Yeah you can build a PSA for about the same amount but there's nothing really wrong with the Ruger. It shoots well and I have found it to be reliable. Most $600 and under AR's are "optic ready" which means no iron sights. The Ruger comes with Iron sights. The plastic Delta ring being referred to makes it easier to take off the front handguard if you need to. It's not top of the line but its certainly not bottom of the barrel either. I like it because for $600 out the door your getting a 1/8 twist, with iron sights, forward assist, and dust cover. That's a pretty good deal. Yes you can build a PSA, but the Ruger name and customer service mean something should you want to resell in the future. A PSA is a build or frankengun. I would recommend it for a fun little shooter. If you want something more "milspec" get a Colt 6920. But your going to jump up in price several hundred more. Good luck and have fun.