I've had both the pairs of sandals you've linked to!
I personally find the Birkenstocks way, way more comfortable. The straps on the Docs aren't adjustable and unless your foot is exactly the same dimensions as the sandal in all three places, they're just not comfortable. Also, they weigh a ton. I sold mine very soon after buying them.
The Birks are easier to break in, lighter and adjustable. Also, you can buy a 'soft footbed' version if you want extra comfort. (They also do a real leather variant if you prefer leather to the Birko-Flor.)
I love all my Doc Marten boots but their sandals are a no from me.