Both are great shotguns that will serve you and the next 4 generations of your family well.
The 870 has far more bling available than the Ithaca, but the Ithaca is a rock solid shotgun.
I own both and don't have a bad thing to say about either one.
Try them both, shoulder them, play with the buttons, rack the slide, and if you can't buy both, buy the one that feels the best to you.
Careful shooting clays...... they're VERY addictive!
sandcast69