You are talking about two very very different guns.
Saiga's are a lot of fun but they can be expensive especially in good mags. If you do want a mag fed shotgun then a Saiga is perfect. If you are mechanically inclined you can do the conversion yourself. It'll be a lot cheaper and you will learn how your guns works.
Now a Mossberg pump is nice if you want something that can be modded out like nobody's business. You can make it into your own and have it be less than a Saiga. If you are dead set on a semi-auto I would look into the Mossberg 930SPX's. Those are nice.
I've played around with my cousin-in-law's converted Saiga and I have my own Mossberg 590SP. The only real thing they both have in common is that they both shoot 12ga, that's it. You should see if you can get your hands on both to try out and make the decision yourself. It sucks but that's really the only way unless you can buy both. Good luck!
Saiga's are a lot of fun but they can be expensive especially in good mags. If you do want a mag fed shotgun then a Saiga is perfect. If you are mechanically inclined you can do the conversion yourself. It'll be a lot cheaper and you will learn how your guns works.
Now a Mossberg pump is nice if you want something that can be modded out like nobody's business. You can make it into your own and have it be less than a Saiga. If you are dead set on a semi-auto I would look into the Mossberg 930SPX's. Those are nice.
I've played around with my cousin-in-law's converted Saiga and I have my own Mossberg 590SP. The only real thing they both have in common is that they both shoot 12ga, that's it. You should see if you can get your hands on both to try out and make the decision yourself. It sucks but that's really the only way unless you can buy both. Good luck!