It's a bit meaningless comparing whether either act has sold out a venue.
It obviously depends on the venue size and the number of nights they play.
In terms of their London audiences, Morissey's has been around 18-20,000 in recent years and Johnny Marr's about 5,000. This is probably representative of the difference generally (Moz 3 to 4 times more popular).
However, Morrissey's fan-base hemorrhaged when he kept banging on about Johnny Robinson and For Britain/Annie Waters etc, probably falling down to about 10,000 (or lower) in London. If that becomes a distant memory and we don't get any more bigoted nonsense, it'll probably return to his normal level, but that's a big if.