I have the Birkenstocks & the Blair dr martens
I find both comfy, I went to Venice and did 4 days of walking in the Birkenstocks!!
Wore the Blair's on the evenings as a bit more dressy - only thing I'd say is try them on and size right, I found the strap was quite big around my ankle and it rubbed a bit.
I should have size down slightly as I have quite skinny feet so I'd recommend trying to them on.
Love them both tho, but Birkenstock most definitely comfier once worn in, they feel so awkward at the beginning!