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.
I'm sure the Syncros rims are good but the DT build quality is second to none. The lighter weight of the DT appeals to me as well. The DT seem like an easy choice except for the price. If budget is a concern, Syncros is significantly less expensive.
From a wheel builders perspective....both rims are solid and are plenty strong for aggressive trail riding. However, i'll certainly agree with the other fellas here and give the nod to the EX 5.1 rims. The reason....consistancy. The 5.1's are consistantly round..rim after rim. Which means less work on the wheel builders part to get them straight and built up with even spoke tensions. When you can build up a strong rim that is round and has even spoke tension then the results are a VERY strong wheel.
Obviously there are other factors that come in to play when building up a durable wheel, but i'd say that having a round rim sure makes all the other contributing factors that much easier.
Another small nitpick about the DS28 .... the rim joint seams on the DT 5.1's are also typically MUCH cleaner.
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