Heheheheheh.
Very good, kleo.
I would tend to think Strapping sells better because it is put out by Century Media and has "genre niche" noteriety - in that people who are into underground metal in general are likely to hear SYL and like it simply 'cause it's metal.
On the other hand, Devy's solo material is on his own HevyDevy records label, and although I believe they have distribution deals through a variety of labels including InsideOut, it probably doesn't get the push that Century Media would put behind SYL in order to push units, as they can ride on City's somewhat legendary status. The solo material seems to me that it would appeal to a smaller, but more dedicated, fanbase.
All of THIS being said, however, I have no idea what the actual stats are. They would be interesting, I think.