I recommend a Remington 870 12 ga smooth bore. I have five 870's, they are very reliable, and easy to maintain. Sights are virtually irrelevant at home defense distances inside the dwelling. An extended magazine may be valuable in an extreme situation and - like insurance - is probably worth the cost. I recommend No. 1 buckshot - definitely not slugs - for home defense. Home defense shotgun loads have been addressed several times here on TFL. For example, see this thread and the reference in the quote:
http://thefiringline.com/forums/showthread.php?t=510474