Hi Jerry,
Here is my take, FWIW. I am a lefty as well and have owned both the SX3 and M2 LH in 20 guage.
I kept the Benelli M2 LH as it is so easy to carry all day in the field and is absolutely durable, shoots when it is wet and/or dirty and is easy to tear apart and clean with little to no tools. In a duck blind, I don't really notice the recoil, but it does recoil harder than the SX3. The only thing that has driven me crazy is twice (out of thousands of times), I have put the shotgun buttstock down as I go over a fence and that small jar has moved the bolt back enough that it wont fire. Now, whenever i put the gun down, I cycle it to make sure. Small thing but it happens. I love the shim system and I shoot the gun exceptionally well.
I loved the SX3 and it cycles FAST. The recoil is also softer than an M2, but not by a whole lot, according to my body. I liked the fact I could adjust the stock for cast/drop so it worked as a left hander. I didn't like the fact that you will catch powder in your face/eyes at some point, especially here in Alberta in late Oct/Nov/early Dec hunts. It is somewhat heavier than the M2, but I also shot it well.
If I only could have one gun, I would go with the M2, and maybe get a wolff +25 recoil spring installed? Still not sure I want to do that yet. If I could have two, I would have both guns and use the M2 for all upland hunting/ugly weather and the SX3 for waterfowl hunting/clays/target.
You will not go wrong with either, I have smaller hands and the M2 feels just right and I love the gun. Also, don't underestimate a lighter gun, especially if you are carrying it all day for 15+ km.