I tried both, using dijon mustard on two Boston butts and leaving it off a third before applying rub/spices when I smoked them all together. I couldn't tell ANY difference between them either with flavor of the mustard or flavor of additional rub/spices that may have stayed on because of the mustard for them to adhere to. Sounds good and looks good on television, but through experience, I can see no practical advantage to applying the "pre-rub".
CMJ - KCBS
CBJ - MBN, NCBS, NCPC
OBJ - BBQCritic.com
CTC - KCBS
Pitmaster and chef - "4 Lil' Pigs" Competition BBQ Team
Author of "A 'Q' Review"
Lang Hybrid Smoker, BDS, Holland "Epic"