I believe the number 1 priority is the guaranteed money. The big bonus total over all the years, so if you offer more per season then someone else for less seasons but his guaranteed bonus total is more from the other offer he may take that offer.
The only problem with offering that kind of contracts is that players 10 years in the league or more have a percentage chance of retiring the next season and this could increase more each season as injuries also can effect it greatly. So if you give that 6 year contract and he retires the next season or before the full contract is finished any bonus money left on that contract will be applied to the next season as dead cap space.