I was wondering if anyone could break down the advantages and disadvantages to Ghost Ring and Rifle Sights on a shotgun.  Right now I only have a bead sight and I want to get irons so that I can be more accurate with slugs at longer distances.  Any help is appreciated!

I find ghost rings faster and more precise. I simply cannot stand the Remington factory "rifle" sights. The Mossberg Glock like rifle sights on the NYPD shotgun are clean and crisp but poorly regulated from the factory IME.

So are the ghost rings on their 930 spx guns, ask me how I know.

To answer the OP's question: I prefer ghost rings as well, to me they seem like more precise and faster sights. YMMV

If you are just going to use the gun at the range with slugs, either GR or rifle sights will work; it's a matter of preference. Actually, with practice, a bead sight can be deadly accurate at distance, as well. If however, the gun will also be used for HD, IMO rifle sights are quicker and unquestionably the way to go. I use a bead sight on my 870 for HD, but when I go to the range to shoot slugs, I switch to a rifle sighted barrel. Another consideration is that, with GR sights, the rear sight is permanently installed on the receiver, eliminating the option of installing a different barrel.

I will be using it for target and hunting (must have iron sights for slugs).  I was not sure if the Ghost Ring sights were worth the extra expense, but it seems as if most people agree they are both more accurate and aid in faster target acquisition.

If I were to buy an aftermarket set of Ghost Ring sights for an 870, what would be my best bet?  How much would a gunsmith normally charge to install them?

I have experience with both the wilson combat ghost ring and the stock remington rifle sights.  Funny thing is the remington rifle sights share the same parts as the irons on some 700's.  

I think both are equal in accuracy, but I'd lean more towards the rifle sights for accuracy, and the ghost ring for speed.  So it's a toss up.

I'd say your biggest consideration would be what are you using it for.  The white painted factory remington sights are sometimes very hard to pick up on certain targets quickly (white triange and white painted post).  The ghost rings don't have this issue.  

I did find the rifle sights a bit easier to adjust to get it zeroed, as much as you can zero a shotgun.  The rear sight adjustment on the ghost rings is easy to over do or move too much.  

Durability, well I've had both come off.  Had the rear blade fall off a rifle sight and lost it (screw must have worked out).  I also had the whole front base come off of the ghost rings, (wasn't staked well).  So even though both are solid, they are not bullet proof.

I will tell you that all rifle sights are not created equally.
I have had two M1s90 shotguns with rifle sights and I was bad ass with them. Texas star would not move, [tight pattern]. Ghost rings on my M2, could never clear a star in 5 shots. Just expected too much. Other models I have viewed did not have the same perfect execution of rifle sights for me and I am currently without a shotgun. A M2 24" Field may be the next try.
Never ever sell a firearm that makes you look bad ass on the course.

I may be the odd man out but I prefer rifle sights on Benellis and Remingtons for the most part. I do like the Scattergun Tech/Wilson Combat ghost rings though. Just a personal preference.

I really like the rifle sights on the Remington shotguns with the replacement XS Express set up.  First off, I really like the sight picture it provides.  It uses a big dot up front with a tritium insert.  The back has a shallow V with a tritium I-shaped insert as well.  In good light, you put the dot in the V.  In low light, you stack dot on the I.  It is faster than a ghost ring set up for me, but I don't personally care for ghost ring sights.  It is also fairly accurate for slug shooting.  It splits the difference between rifle sights and a bead.  So I endup with sights that are fast and accurate, which is definitely a good thing for shotguns.  

Secondly, I like the fact that the sights remove with the barrel.  So I can remove the rifle sighted barrel and stick my bird barrel on it.  I can also put on a beaded barrel if I choose.  With a ghost ring set up, the rear isn't readily removable. So the rifle sights w/ the XS Express set up is more flexible for me.  It can be removed and replaced without loosing it's zero.

On the down side, it can be pretty expensive.  Unless you already have a rifle sighted barrel, you need to get one.  Then you have to add the XS sights.

If you talk to ebnough shooters you won't really find that to be true but it is really about preference.  I prefer never to use Ghost rings, too slow.  Second, I prefer never to have a rear sight on the receiver.  If I have a rear sight at all ona gun, it is on the barrel.  A bead is by far the fastest sight due to being so minimal but it requires practice.

If you choose a barrel with rifle sights, you can still swap barrels for other purposes without having that rear peep sight on the receiver.

for 3 gun, I shoot 10/22 flip ups with Hi Viz triangular front (green light tube)....a true ghost sight must be mounted near your eye on the rear of the receiver... and this may be a problem for aftermarket installation...I find that I can look over top of the barrel and get a "good enough" sight picture with the rifle sights for poppers and then refine the sight picture for plates or slugs... I find that I can shoot weak hand/eye better with the rifle sights, but I wear monovision shooting glasses

My descending order of preference:
Wilson Combat Ghost Ring with front tritium
Rifle sights
bead

CS

I also like Ghost Ring sights over the bead. I'm more accurate with the GRS than a bead but both are fast. To me the big benefit of the rifle sights is having the sight system on the barrel and not on the reciever.

My HD weapon is a Benelli M4 with Ghost Rings Sights.

Factory Remington with XS upgrades