I have a feeling that nosedive’s 521’s are the ‘old’ 521’s if they are more 6 years old. For some reason Mavic updated all their range, keeping product numbers but moved rim specs around – so the ‘old’ 521 became what is the 721 today, which is my default DH/heavy duty rim that’s hard to beat.
EX721 is the DH rim – a more noticeable 590g verus 470g of the black Flow. 521 is the trail rim that compares to the Flow.
Pros/Cons of Flow v’s 521 already posted – I’ve dinged my Flows already, surprisingly easily, but the tubeless is still sealing and holding up well.