mossberg 500 series vs rem 870

[Flashdragoon]

So I am getting my lady a shotgun for home D. She is quite experienced with 12 ga target shooting (clay pidgeons and the like) but neither of us have really considered a shotty for home defense until recently, I've narrowed what I like down to 2 possibilities:
This remington 870
Remington Model 870™ Express® Synthetic 18"
and this Mossy 500 tac
O.F. Mossberg & Sons, Inc. - Firearms, Shotguns, Rifles, Accessories, and Precision Machining
(on the mossy scroll over till you get to the 500 tac 6 shot

Now bear in mind I personally know nothing about shotguns, I hear talk of smoothbore, choke tubes, sabot rounds, buckshot, rifled slugs and the like but know naught of what that means. I am not looking for some fancy tricked out to the 9's shotty just a good reliable shotty with a short (legal short) barrel as my home is cramped in space. Something that will cure the bump in the night worries, anyone familier with both of these weapons? recommendations? anything to steer clear of?