First, a 20" rifle is a great one to have. Today, many people like the 16" carbines. There are benefits to the shorter rifles, but the longer rifles have their pluses too. First, the cycling of the 20" rifles is smoother than the 16" rifles. It's hard to believe it makes much difference, but I can certainly feel it when shooting them side by side. Second, if you use iron sights, you get a longer sight radius, which makes aiming easier/more precise with the rifle length. You also tend to see less cycling issues with a rifle length.
I can't speak of the Wilde Chamber much. I used it for a while on one upper, but I've never shot anything but .223 cases out of any rifle so I really haven't seen how it handles 5.56.
For twist, there is always an up and a down to any twist rate. 1:8" is a good twist. It lets you shoot into the 70-80gr bullets and still is capable of shooting 55 or 62 gr bullets just as well. The down side is that if you plan to shoot 36-40gr bullets you may have to back down on velocity to keep them from coming apart. Not many people are looking to push those light bullets slower either. Some people claim you can over-stabilize lighter bullets like the 55gr or 62gr out of a 1:8" or 1:7". I've never seen it. I've seen some rifles shoot 55gr stuff well and others not so well, but I've seen it happen the other way too where even with a twist that stabilizes the heavier stuff some just wouldn't group well. Personally I put the over-stabilize idea as myth, until it can be proven by more than one guy saying his rifle doesn't shoot them well. Many claim the faster twists cause more friction and with that more heat. They say a faster twist barrel can be shot out faster than a slower twist and to only use as much twist as you need to stabilize the bullet you want to shoot. I'm not sure if this is a realistic issue or not, but it is something to consider.
If I were to buy a rifle length upper I wouldn't hesitate to buy it in 1:8" or 1:7" twist. Most of the drawback is not being able to shoot the ultra light stuff, which I'm ok with.
I'm not sure what style upper you are looking for but I might not go with RRA depending on what you are looking for. If you want one of their varmint uppers, I suggest you look at WOA instead. They make some great shooters for a very reasonable price. Many people have seen 1/2" groups at 100 yards from them.
If you want a standard rifle length setup BCM makes a decent upper for the price and they tend to not cut many important corners when it comes to making a reliable AR.
Really from there, for accurate rifles, RRA then offers good uppers with a few different options. I may also buy the parts individually if you are looking for a specific setup. I probably wouldn't use an RRA BCG, just because getting one that has been tested like the BCM runs identical money. I'd like the extra assurance that the BCM bolt gives.
Other than that, even if you go RRA, you should have an accurate fun rifle. Enjoy the thing.