Andy,
While I have had a lot of experience with other firearms I am relatively new to shotguns.
However, I went through the process of looking at SGs recently. I purchased a Winchester 1300 Defender in 12gauge about 3 months ago.
It is a GREAT gun.
I had to, like you, consider the recoil problem because of my wife. Regarding 20gauge and 12gauge. I did the same thing and looked at the two. I am 5'11" and about 225lbs and have shot hundreds of thousands of rounds of various types in my life. My wife, on the other hand is 5'4" and about 118lbs, and very in-experienced. This is why, I too, was looking at the 20gauge. After shooting the two I became convinced that the 12gauge was the way to go.
Labgrade is right the 20 is light and with the alloy receiver in the 1300 this makes a light gun even lighter. The 12gauge comes in at 6.5lbs and the 20gauge a 1/4lb lighter at 6.25. I shot both guns (and an 870 Wingmaster in 12gauge) prior to plonking down the cash, and I was surprised at the kick of the 20.
My 'little' sweet heart has no trouble with relatively light 12gauge 1 1/8oz target loads (100 packs from Walmart).
She also had no trouble with the standard 9 pellet 00 buck.
However, she much prefers the 8 pellet reduced recoil offerings.
We both felt that the Federal magnum 18 pellet #2 buckshot in 20gauge kicked more than the reduced recoil 12gauge 8 pellet loads!!
She tried one 3" 1oz. magnum slug and would have hit me if she could but she busy rubbing her shoulder
Now don't get me wrong a 12gauge weighing in a 6.5lbs with a 3" magnum 15 pellet 00 buck kicks like a 'son of a female dog'. I would only use those in bear country... or nasty big pig country...
Another challenge of the 20gauge is getting shot larger than #2 buck. For home defense I don't like the idea of using anything much smaller than 00. There is just so much more ammunition available for the 12gauge!
So with reduced recoil loads for slugs and buck shot in 12gauge,
IMHO for versatility, availability and ease of use I would suggest the
12gauge
Then again nothing will compare to actually trying each of the guns for yourself...
Regards,
HS/LD
BTW I have over 2500 shells through my Winchester Defender without the slightest problem of any kind. I have posted a couple of reports (with pictures) on this forum about the gun I hope the one you get performs as well as mine