you are bound to get feedback from knowledgeable folks than I (ie, Squeaky Wheel) on this matter but my opinion is that the 5.1s are the way to go.
The DS28 are great rims for sure. However, the way I see it is that both are 28mm wide (the Syncros are taller), the 5.1 rims are about 40 grams lighter a piece, and the DT Swiss tubless conversion kit is fantastic and it is apparently now available for the 5.1 rims. I have 2 sets of 4.1 rims and the tubeless conversion kit works perfectly.
The DS28 are great rims for sure. However, the way I see it is that both are 28mm wide (the Syncros are taller), the 5.1 rims are about 40 grams lighter a piece, and the DT Swiss tubless conversion kit is fantastic and it is apparently now available for the 5.1 rims. I have 2 sets of 4.1 rims and the tubeless conversion kit works perfectly.