A Mac generally comes with fairly powerful hardware, which is important for any development machine, and might make an observer think is OS is the good part. However, I remain a die-hard Linux junkie. MacOS probably isn't any better for web dev than any machine running any other OS. The power of the computer is in what you do with it.
My history:1995: started using Linux as a hobby dev OS
2000: switched from Windows NT 4.0 to Linux at work (C)
2002: moved to .NET, used Windows at work and Linux at play
2006: mix of Linux and Windows at work
Mac Os X Vs Linux For Development System
2011: abandoned .NET entirely; mix of Linux and OSX at work, Linux for play
2013: entirely Linux
2014: mix of FreeBSD and Linux for work and play
So it's fair to say I've solid experience in both, plus a fair amount of dev environment automation under my belt.
Linux vs OSX: Linux wins hands-down unless you're happy hitting a wall when it comes to customisation and automation. Mint is in my experience the equal of OSX out of the box, and just gets better as you start automating things. Productivity with a decent keyboard-driven WM and editor (StumpWM + Emacs in my case) is great.
Mac Vs Windows Vs Linux
Recently I've become a bit concerned that some of the design directions taken by the Linux community aren't aligned with my priorities, so I've been trying out FreeBSD. It's awesome, and my current OS of choice, but hardware support is more limited and the desktop / laptop experience isn't as polished as Mint.