1. Is that a good gun (price range) compared to the Browning Gold/Silver, Mossberg 935, Franchi I-12, Stoeger Model 2000, and the Beretta Urika?
That really depends on how it fits you. The Franchi fits me a lot better than the Urika/Browning/Mossberg. Fit is a very important thing to consider here.
2. Is the gas system better than the inertia systems?
Yes/no. The gas system will take away more felt recoil (usually). But the recoil operated guns run a little better. IMHO
3. Does the difference in the 28" barrel and the 26" barrel a noticeable difference?
For a field gun, not really. You can learn to shoot either well. For a field gun, I prefer a 26". And for clays, my gun sports a 30" barrel.
4. Will the 11-87 Super Magnum shoot ALL rounds? Does it shoot the lower brass shells? I heard someone say the gun comes with 2 O-rings that can be swapped and both shells can be shot...I dunno...
I can't help you out much here. I only have an 1100, and it is 2 3/4" only. Best to check with Remington for a difinitive answer. I would suggest dropping them an email, or a phone call.
Bottom line, I would go and handle as many options as you can. See what one shoulders/fits/swings the best for you. And good luck.