The difference I think Drew is that Mahmood is a young inexperience bowler who at times looks world class no matter what the surface, so if he improves his accuracy and control will be a good addition to the side.
Lewis on the other hand, is pretty much bowling as well as he can, and has only looked effective on home conditions, and when he's bowling in your standard English weather. I did think he would be able to adjust, but the Aussies in particular really look like they are going to go to town on him, even in that last match against New Zealand the pitch was actually more suited to him than Jimmy or Fred and he was outbowled. The reports from the academy were also bad with apparently every team they came up against taking a liking to his bowling. With the consistent bounce the Aussie pitches offer and with there sometimes being no swing on offer, he doesn't have enough about him to still cause problems.
Might turn out to be a horses for courses bowler, as if there is cloud cover, or some seam movement, or even in England, he is a very different prospect.