Specifically, a bull barrel is heavier than a standard barrel. That is neither a negative or positive, unless you're carrying it over your shoulder for miles on end, but will result in somewhat less percieved recoil, so you can judge if that's a positive or a negative.
A bull barrel has an 11 degree target crown--postban version--(which typically will result in better overall groups) as compared to a preban version flash hider that will disrupt the gasses propelling the bullet as it exits the bore.
A bull barrel along with being heavier, is more ridgid, so it will have less "barrel whip" when the bullet is traveling throughout the bore (and will take longer to heat up so there will be less stringing possibilities). however, if you overheat it, it will also take longer to cool, so, in all, that could be one up for a standard barrel if you plan on repeatedly running it hot. Just keep in mind, heat is a bad thing when it comes to barrels.
Fluting is another option to make a bull barrel even more ridgid. I've also heard both positives and negatives with cryogenically treating barrels, so I can't say if that will help or hurt you. I personally don't have any firsthand experience with this method, and I am going to leave the hearsay offline and will try to keep this post with only the facts.
Freefloating as mentioned earlier, is a big big plus with any type of barrel so you won't be creating moving pressures on the barrel whether it is with a sling, your grip on the rifle, etc.
There are a lot of bull barrels that front sights are offered as standard equipment on them. The only difference is the size of the hole the barrel slides through is a larger diameter on a bull front post. The post height is still the same. Or you can get a picatinny gas block and have a removable front post.
If I were going to have a flat-top reciever for target only, I would prefer to use a magnifying optic, and eliminate the front sight altogether, but if I were wanting it for home defense purposes also, I would want one with a front post, so it may be best if you purchased 2 entirely different upper recievers and swap them out depending on what mood you were in that day, but if you have a quality optic, the front sight won't interfere none whatsoever with your sight picture through a magnification scope.
bull barrels as stated earlier, doesn't necessarily have a higher twist rate. I have seen some offered at 1:12 twist rates, but typically, even the bull barrels still have at least a standard 1:9 twist and won't offer any better accuracy from the twist rates alone. That really doesn't have anything to do with accuracy. The bullets you plan on shooting will determine that, so I can't really agree with that statement above.
Basically a 1:12 twist will shoot the lightest bullets with the most accuracy, 1:9 will shoot the average available 45-62 grain ammo the best, and the 1:8 or 1:7 twists will shoot the heaviest bullets the best. A lot of accuracy will depend on what you're feeding through it. It's best to experiment to find which load, weight, and brand will perform the best with any rifle.
If you're going to get a bull barrel and you're wanting the most accuracy you can squeeze out of it, I really feel you should consider a broach cut bore bull barrel by Olympic Arms.
Since most brand bull barrels are still a button rifle cut (the same method as any standard barrel of most other brands), the barrel itself other than being thicker won't really improve the accuracy except for less barrel whip generated.
Olympic Arms (SUM--stainless Ultra Match) barrels are broach cut and are superior to everyone's bull barrels simply because of that reason.
In other words, since most other brands bull barrels are rifled the same as their standard barrels, the only thing you're going to get out of it is more ridgitity. It helps some, but the broach bull's will out shoot the others all day long.
As far as what length to get, for your post mentioning target plinking and home defense, if it's within 300 yards, you'd be alright with a 16 inch barrel. If you're planning on shooting at over 300 yards, get an 18-20 inch barrel. If you're like me and enjoy pushing the envelope and shoot very long ranges well in excess of 700 yards every chance you can get, buy a 24 inch barrel.
I know this post has been a long one even for me, but I hope it helps you out. I've been doing this a long time, and if you have any questions, just drop me a PM or email. I'll be glad to help you out.
Oh, and the gas block is either built into the front sight post, or is where the front sight should be if you have a "shaved" or picatinny gas block with a rail built onto it.
There is a gas tube that runs underneath of the front handguard that connects to the front sight (gas block), and that's what causes the rifle to cycle.
If you buy a complete upper reciever or a complete rifle, it will already be assembled and you won't ever have to worry about it, and yes, you can exchange them for different versions if you don't like the one that's installed on it.