Carter can still make good money and create something great with this young talented supportive cast. If he wants to be the alpha male and get the max FA contract, then should probably let him go, rather than using the franchise tag.
Could make the same argument regarding Murray, but may franchise one or the other.
You always hate to lose a guy you drafted and developed. Especially if it took a couple years ot get the guy to a level worth spending on.
But at the same time as you mention, these guys are about to hit the free agent motherload and seldom do they want to leave a lot of money on the table having never made much as non 1st rounders.
I don't think Dallas can afford Durant, Carter and Ro plus have Sean Lee.(And Lee costs more to release than to keep for 2015).
So I am guessing one guy walks and Carter seems the most likely to get overpaid.
I'd be really tempted to franchise Murray and pay him the huge 2015 salary. RB can be so very good but also for so very short a time period. Guess in the end it isn't much different then giving him a Marion Barber contract knowing you will eat a huge amount to cut him but never paying a high cap hit until you do.