Neither are an adequate substitute for regular strength training. They have their own benefits, but don't replace the benefits of ST.
THe problem is, if you can't do weight because of your shoulder, then you won't be able to do yoga, either; there are many poses that place a lot of strain on your shoulder.
My bet would be to talk to your doctor, and get a referral to a physiotherapist who can design a program around your injury, and ensure that you don't exacerbate it.
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