I had the same problem when I purchase 3 soda kegs. After replacing the o rings and letting the kegs soak top to bottom in starsan for a couple of hours, a little of the smell still seemed to be there. Regardless of the smell, I pressed on and kegged my beer.
Guess what...the beer smelled like beer and tasted like beer. No soda smell or taste were in the keg after the beer was gone.
My rule of thumb is if you cleaned top to bottom, regardless of the sanitzer, and you still smell something...it's mental. Don't worry about it. If you still are worried about it, go back the dreaded bottles.
Good luck in your kegging adventures!