IMO, I think Mario and Smith's concerns are that generally a 3-4 DE is big, around 285-300+, Richard Seymore type players. They also get double teamed quite often, in fact, that is their job, to take on as many blockers as possible so the LBs can make the plays. I believe this was one of the reason's Smith left Arizona in the first place, because they were switching or had switched to a 3-4 when he is a 4-3 DE.
I can understand their concern, I think it is putting the cart before the horse though. I also think both would be successful in a 3-4. I could also see Mario standing up and rushing the passer. I remember during his 14 sack season saying he liked being in a 2 pt stance because he could see everything better and could locate the ball better. Now maybe Mario's concern is because they want to move him to DE but I think he would have no problems if he played OLB.
just thinking outside the box a little....