Anyone who gears correctly will have a Stat/Attack ratio averaging 4:1. With that rule of thumb, you can look at a medal or item as just know which will be better. It's going to be very character specific between the medals in question, but with a standard ratio between 4 and 5 to 1, the Thief medal definitely loses to the Next Legend (for non-mages). Of course both medals require a lvl 200 characters. Instead, you can go with the Silent Crusade top medal which is +5 all and 1 Attack. It only requires you to finish the first two chapters of the silent crusade, and is thus far easier than getting to 200. Most of the 200 medals are crap, unfortunately. Wiseman and Hero's Successor are good, though. For non mages, the Next Legend is a great medal. But for Mages, the Silent Crusade medal will beat it easily.
TL-DR: My comment of the SC medal being better than either of the other 2 is unqualified. It is better for mages, by far. For non-mages, the Silent Crusade or Next Legend is definitely better.