Unmodified ("sporter" stock) Saigas point poorly, handle like crap and have terrible triggers.[/QUOTE
The good news is that if you own a drill, a file, a screw driver and have an internet connection there is no reason you have to leave it stock. A converted saiga is not comparable to a stock one and anyone serious about using their S12 converts it.
That means you can't leave the Saiga loaded and forget about it like an 870.
A magazine in my experience can be left loaded for months without any problem. That is as long as I have left any loaded so I cannot comment beyond that.
If you are talking about leaving the mag loaded on the closed bolt this can be an issue but there are so many simple solutions that it is not really an issue. The simplest is to leave the bolt back. Since an 870 is not drop safe I would not leave one chambered anyways so one is at no disadvantage to have the bolt back. It can be dropped in literally a fraction of second.
If that is not an acceptable solution then one could turn to ammo selection. Not all top rounds will deform. Some will but not all. Thus if one is aware of the issue and wants to leave a round chambered and a mag loaded then one can select one of the rounds known not to deform as their top round or fill the whole mag with the stuff if they like.
The bolt can also be modified in a way that will, as reported by some users alleviate shell deformation.
I think there are things that need to be addressed on the S12 but shell deformation is honestly not one of them and is something that gets brought up by people too misinformed to realize it is not really an issue.
I have multiple 870's and a mossberg so I don't really have a dog in the fight but it does get old to hear the same old second hand (and incorrect) stuff re-posted again and again.
There is no reason a S12 can not be used by an owner up to the task for HD. That said I am not sure that many of the S12s advantages will be factors in most plausible HD scenarios.
I would prefer a 870 to a stock S12. I'd prefer a well tuned S12 to a 870. A well tuned S12 will take a fair amount of time and money over the stock gun. You are probably looking at benelli money to do it right.
A 870 with a few add ons such as a light and a side saddle is a much cheaper path to a perfectly fine HD shotgun and the extra money could be used on training, which will have a much greater effect than what type of gun you have (presuming they are all reliable and will go bang).
That said you have an S12 so here are some things to be aware of. Those 8 round mags are not legal to use in an otherwise stock gun. If you want a good HD gun then there are some modifications that will greatly enhance the S12. I don't have time to chronicle what I believe they are right now but I will try to do it latter.