Remington 870
Why?
1. Robust/reliable. The police use them, and while this usually means 'made by lowest bidder,' in this case it means 'able to take 20 years of crippling abuse and still go BOOM on demand.'
2. Economical. The police use them. This means that if its not made by the lowest bidder, its damn well close.
3. Smoooooooooooth Action. Yes, semis are faster, but not by a whole heck of a lot.
4. Accessories out the arse.
5. Simple manual of arms.
I would go with a pump over a semi. I don't care how reliable the semi is, it is FAR more likely to hiccup than a decent pump, especially if you use non-standard ammo, are not keeping it clean, or- in the case of recoil operated guns- doing things that affect recoil.
I would go with a pump over a sxs. sxs's are nice, but by definition you get 2 tries before you need to reload.
I would go with the 870 over the Mossberg, but this is personal preference. I think the 870 is exceeded by the Mossy in no area except maybe economy, and the 870 is (IMHO) more durable. JMO. The mossy isn't bad, though...if I had to use one in lieu of an 870 I would not feel poorly armed.
Mike