I've never seen a firearm with more accessories and interchangeable parts (and as high an availability of the aforementioned) than the Mossberg 500. You can get barrels anywhere for good prices.
The ithaca is a gorgeous gun, and the fact that the shells eject from the bottom makes it an easier to use gun with less chance of fouling, in my opinion. It costs more, but it holds its value. Also, I'm not sure which models feature it, but some of them have wonderful engravings on the receiver that I might pay extra for if I had the money just to make it a heirloom and something you are proud to own. But that's just me.
You really need to go to a shop that sells all three and pick them up and see what feels "right". All three guns are classics with millions of happy users, you're not going to go wrong.