Glad to hear you've fallen for this, I am a recent addict as well.
I personally shoot an 870 pump with a modified choke, on a skeet range.
What I have found, is a general rule of thumb is people shoot with Cylinder, or Improved Cylinder, and a #8 or #9 shot. Gives you a super wide pattern, with tonnes of shot to hit the target. The tightest most go is Skeet choke, but I do see the odd O/U shooter with an Improved Cylinder and Full choke.
I personally enjoy the challenge of shooting Modified on the lowest number allowed at my range, #7.5 shot at 1oz load. I do this for 3 reasons...
1. I like letting the clay get way out there, and shooting under it. Good challenge, and requires a lot of calculation.
2. When the target is close to medium range, a direct hit OBLITERATES it! I love watching the clay turn is dust in the air, that's when I know i hit it smack on. Cylinder and Improved cylinder shooters get that effect, but mine feels like it is epic!
3. #7.5 has less pellets at the same weight load #8 & #9, so it poses for more of a challenge. When you do decide to open up your choke, and drop your shot size, you'll find it is easier to hit the targets. Kind of like running laps with training weights on. Take em off after training, and you'll soar for the competition. Simply because with the tighter choke and less pellets, you're forced to be a lot more accurate, and in the competitions all you need is for the clay to deflect. However, some will say to shoot with the same setup that you practice with, for consistency's sake and whatnot.
All that, and on top of it, my shotgun came with the modified choke and I've been busting them since day 1, so why not.
Oh and about busted up shoulders... I have a light weight 300winmag rifle that i shoot off, before blowing through 100+ clays, and I feel like tenderized meat for 3 days after =D Limb savers, yeah, they do the body good. Cant imagine how that plastic butt plate feels.
Anyways, welcome to the dark side. May the Clay Busting Force, be with you...