Good morning. I'd like to weigh in with info on this subject. I've been a soldier off and on (currently on) since 1986, going through the M16A1, M16A2 and currently the M4 military versions of this rifle. I also own several civilian ARs in both carbine and rifle configurations.
As to the first question, why?, my response is twofold: First, I own these rifles because I can. A right, like a muscle, must be exercised to be of any utility. It is important to me to be able to own weapons, not guns. Second, I just love to shoot. I use my ARs in competitions and just for fun. My wife and kids can shoot them all day and have a great time. They are accurate and easy to operate.
As to which AR, there is no single answer. First, determine what you want to do with the rifle. If you plan to shoot at 50-150 yards, I'd suggest a 16" carbine. If you plan to shoot across the course matches, I'd opt for a 20" rifle. My preference is for either a shorty stock, or an A1 length stock, as I find the A2 to be too long, even for my long arms.
Any of the major brand names create a fine weapon that will last as long as you want. For my money, even though my issue weapon is a Colt, I would not purchase a private AR from Colt. They are overpriced, and offer no perceptible benefit. A Bushmaster, Rock River or any other major maker will give you more rifle for the money. Also, there is no need to swap triggers. While a NM trigger may be smoother, with proper practice, the stock trigger is more than adequate. I have been qualifying expert with stock triggers for 20 years in the Army, and shoot just fine with my personal rifles, too. If you plan on Camp Perry, then a new trigger might make sense, but certainly not for casual shooting, or even three gun matches.
For sights, it is hard to beat the factory iron sights, and that is where any rifleman should start. Once you are proficient with those, not before, a red dot is awfully nice. I use an Aimpoint Comp on my service rifle, but an Eotech on my personal rifles. Either is a great choice. An ACOG is also fantastic, but I haven't been able to justify the cost.
As for what I actually own, in order of their use, I have the following:
S&W M&P
Bushmaster carbine
Bushmaster 20"
Rock River carbine.
Good luck.