Yknow, for a hot second there I was confused at all the people saying Nolan was anywhere close to Boyd, because my last Boyd was an unkillable god who no one except maybe Gatrie (who I baby the hell out of, 'cause he's my bro) and Aran/Nephenee could compare to.
Then I remembered that I gave Boyd Vantage, and he barely took a hit for the rest of the game. That was a trip.
Anyway, long story short, I don't think any of the DB are explicitly better than the Greil Mercs, though it does depend on how you take care of them. My Laura and Rhys have wound up pretty equal in almost every run, as have my Aran and Nephenee. There's no reason not to use DB over GM, but using all of them is probably gonna put some of your Laguz units at risk, and Janaff is too precious to me for that amount of snackrifice.