Trade away Calvin Johnson for Antonio Gates?

I'm weak at TE (technically don't have 1 right now) and by this Sunday I'll have somebody like D. Keller, T. Moeki, J. Gresham, or T. Heap. My league requires we start 3 WRs and 1 TE. My WR corps is Fitzgerald, C. Johnson, M. Floyd, J. Knox, D. Bowe. Guesstimating from a math standpoint, the weekly point drop-off of losing someone like CJ compared to my other WRs seems to be about 3 points per week based on past years stats. Gaining an elite TE like Gates (who is basically just a big WR) over starting a weak TE every week seems like it may gain me slightly more than 3 points per week. So do I accept the trade? Best answer will be someone who best explains why or why not to make the trade.