I voted Mark Joseph but actually they're both nice middle names. If I had to choose one as a FIRST name, I know I would choose Joseph so that's why I chose it, but as a middle name I can see what you're struggling as I like them both

...which means I'm no help at all!!
14 days to go (probably less)...decision time!! Other option is, what is YOUR favourite.... when I had my emergency c/section with our 2nd, I'm being wheeled up to theatre and dh says so what's our definite middle name for a boy?.... I said I don't know but gave him the one I wanted and he just said yes that's fine..I think in the throes of it all he was just happy to let me choose!!!!! So you could wait til Mark's about to make his arrival and tell dh what you want haha - just kidding!