I am looking for an all around shotgun(hunting, clays, defense, fun). I really want a Benelli M1S90. My first question is:

Is this a good platform or do you think I would be better suited with a pump action or perhaps an M3 Pump/Semi? Take into consideration that I am most familiar with semi-auto rifles and am not used to having to pump the action for every shot.

Second question: Should I go with ghost rings or rifle sights? What is the advantage/disadvantages of either?

Last question and the one I need the most help with: I want to be able to shoot light game loads for practice(#8, #6) and clays...as well as use OO buck and slugs for defense. I am really confused when it comes to chokes and slugs. I use to think that 00 buck was the king of the defensive round but the more I read on the net...seems like more people are using slugs. The Benelli comes with an IC choke, Full and modified.
---What are each of these chokes used for and which one is best suited for defense, hunting, clays?
---Do I need a special barrel to shoot slugs in a Benelli M1S90?
---Can someone explain the different types of slugs(sabot, rifled, tactical etc..)?
---What is the advantage of slugs over 00 buck
---Do I need to constantly swap barrels or chokes when shooting slugs, buckshot or game loads?

I know this is alot of questions but I am getting into this shotgun thing pretty deep and am in unfamiliar territory. I would appreciate any comments, criticism or helpful advice. TIA

Quoted: I am looking for an all around shotgun(hunting, clays, defense, fun). I really want a Benelli M1S90. My first question is:

This post could go for months.

"Hunting, clays, defense, fun".  Is that the order of importance for how the gun will be used?  If so the M1S90 may be your worst choice.

To be over simplistic, buy a Mossberg, Remington or other pump with 2 or 3 barrels.  A 28" vent rib barrel with choke tubes.  A defensive barrel 20" improved cylinder, large bead sight, no choke tubes needeed.  Third, a rifled slug barrel for sabots.  Same goes for any semi-auto.  The first 2 barels use the same basic bead set up, allowing you to work on technique as opposed to dealing with different sights.  The slug barrel is scoped.

The Benelli M1 is available in tactical 18" barrel configurations and field varieties with various barrel options.  Mine was originally an 18" with straight grip and rifle sights and it is used in that configuration when shooting Limited class or tactical matches.  I have a 26" ribbed barrel which is use with red-dot optics in Open Class.  Both barrels are smooth bore.  Rifle sights are quicker for me then ghost rings but ghost rings are probably more accurate for longer slug shots.  Neither are very good for clay bird or hunting and in fact I have ordered a 21" ribbed barrel to replace the 18".  Choice of choke depends on the expected range of the target when shooting shot.  Full is suitable for trap but maybe a litle tight for the 16 yd line.  Modified is a good all around choke suitable for sporting clays and hunting.  I use IC for close range work knocking down steel and stationary clay targets.  Sabot slugs are desisnged for rifled barrels, the conventional forster work in smooth bores.  Low recoil slugs may not cycle, see my previous post regarding ammo for M1s.  Most people recommend the most open choke for slugs, this being the IC.  I use modified with my loads and think it is a little more accurate.
Choice of slugs or buck for self defense depends on the expected range and target.  00 buck with a full choke will shoot about a 6" pattern @ 25-30 yds.  I keep shot loaded for home defense purposes expecting in house shots.
An 18" barrel is a little short for most clay target games although I've had some sucess shooting trap in my tactical configuration.  Switching to a 26" for this probably is the way to go or just buy a 26" gun to begin with and accept that it will be a little less handy in tactical situations, either games or real life.  I have thousands of rounds through my shotgun and would not consider another auto.

Go to WWW.REMINGTON.com, click on support, click on reading room, to Shotgun Guide. Required reading. Mossberg also has the same type of material on their site. MIKE.

I had a customer in a similar situation.  I ordered him a M1S90 tac with rifle sights.  This way he gets side sling mounts, mag extension and a 18.5" barrel for home D.  I also ordered him a 28" vent rib barrel so he can go bust some clays and go hunting.  
IMHO ghost rings are a superior sighting system for 'combat' shooting, but are not required.  
I thought about getting an all purpose shotgun setup but decided that my M1 was going to be my go-to shotgun and did not want to compromise.  However I also have another shotgun I can hunt and shoot clays with so it was an easier decision for me.  Hope that helps somewhat.  

What Zoub says.  Get one of the Rem/Moss/Win package deals.   28" vent rib and a 20" slug barrel.  Experiment with that set up for a few months.  If you decide you like the shotgun game then maybe consider moving up to the Benelli, if that's what you want.  Frankly, I think Benelli's are neat but over priced.  I'm not convinced they put out bird shot any better than the others.

The package deals will only set you back about $300 or so.  A Benelli with all the bells and whistles will be pushing a grand or more, depending on which flavor you buy.