i think the 5.1 modified set bonus is more appealing than the original 3 tw bonus. i dont really see a reason to not use it unless you had a higher ilvl heroic piece to put in it's place.
IMHO, the 4pc-bonus isn't worth it. I have it. I used it once, then swapped back to my non-set stuff. And as soon as I get the right off-pieces dropped, I'll be wearing shoulders and helmet.
To be honest, I found the old version of the 4pc-bonus more appealing, but it still wasn't worth wearing all that badly itemised armor.