Imo:
DASH is just mixing. Its not really any more private than a bitcoin mixer.
Monero ring signatures offer far more fungibility/privacy than DASH, especially once the ring confidential transactions feature is finished later this year.
Also DASH had an instamine and is heavily owned by one entity. The distribution of Monero was very fair, in fact, its probably the fairest distribution of any proof of work altcoin in the top 20 of CMC.
Also there is this:
https://twitter.com/petertoddbtc/status/622022840330682368Quite a number of prominant bitcoin names have said good things about the cryptography in Monero and bad things about Dash.
For these reasons I think Monero has a lot more potential than Dash.
Further, when I see Dash at 2-3x the price of Monero, and I see that the trading volume of Monero is much greater than that of Dash, I believe there is a severe market inefficiency. Monero should probably be 3x the price of Dash, not the other way around.
One interesting thing about DASH is that the masternode system is actually pretty similar to Bitshares DPoS. This polarizes people into two camps: the people who only think PoW is secure, and those who like systems such as Dash masternodes, Bitshares, PoS variants, etc.
I figure that by holding Bitshares I have the proof of stake variant part of my portfolio covered, and for the sake of diversification I had better hold a solid proof of work coin, in case the peopel who say proof of work is the only way and every PoS variant is flawed are actually correct. So for that reason also I prefer Monero over Dash, because it is much more different from Bitshares and thus helps me to better diversify.