As I tried to explain, both systems leak some entropy about previous transactions, but there's much less leakage in monero than in DASH (that, under the assumption that the dark nodes are not keeping the information that *totally* exposes the link).

The amount of information that can be extracted depends essentially on the anonymity set.  In as much that being member of the anonymity set is mandatory (as far as I understand, ONLY monero (as a representative of cryptonote) has this property - I'm not sure about ZCASH), this has the advantage that the anonymity set grows over time even if you do nothing.  With an optional anonymity, such as in DASH, it depends on how many users actually use it.   But both in monero and DASH, the anonymity set is (initially) finite and even rather small.  In DASH, the anonymity set is guaranteed to contain only other people wanting anonymity explicitly ; in Monero, the anonymity set is random and growing.  But both sets are small in the beginning.

In ZCASH, for anonymous transactions, the anonymity set is much larger, and essentially equal to all people ever having done at least one anonymous transaction.   However, in as far as ZCASH has no mandatory anonymity, that set can in practice turn out to be much smaller than the anonymity set in Monero after a long time, where this will tend to include all users of the block chain.  In as much as ZCASH would have mandatory anonymous transactions, it would be the only system with TOTAL anonymity instantaneously (that is, the transaction can be just ANY person on the block chain).

This is why this question of mandatory anonymity for ZCASH is so important: if it *is* mandatory, it is the only system that is FULLY anonymous right away.  If it isn't mandatory it has a similar problem as DASH.  It would be a pity.  (it would still be better than DASH because of the lack of need for master nodes)