I shoot both 12 and 20 bores. My go to gun for clays is my Browning 725 in 12 bore weighing in at around 8 1/4 lbs., OR if my shoulder is giving me grief, then my Beretta 303 auto in 12 bore, which is around 7lbs.. My 12 bore choice of cartridge is 24 gram, make is irrelevant. For most of my game shooting and occasional clay shooting I have a Browning 20 bore weighing in at around 7lbs. As Will said, it is harder work, it's lighter weight is easier to stop and you have to work at the swing, even with 30" barrels. My advice is get a 12 bore, a far greater choice of cartridges and around £40/1000 cheaper too I disagree with the 20 bore having greater recoil though, I restrict my cartridge loads to 28 grams or less. If I needed more than 28 gram of shot, I would use a 12 bore, stuffing 32 gram through a 20 bore, to me is a nonsense.
Forgot to say that I have instructed many 100's of beginners in the past 40 years or so, for corporate days with almost all none shooters. The sole purpose of the exercise was for them to hit at least a few clays. My choice of gun was always a Beretta field model auto, it would cycle 24 to 28 gram cartridges all day without a hitch and I am safe in saying not a single injury or bruising to any of them, Male or Female, young or old. they all came back for 'seconds' !