Both are in great demand right now and a quick study for anyone with a semi-technical background.
I recommend plumbing b/c the A/C path leads you into extremely hot, dusty attics et al during the summer months (and you're not getting any younger!). Also, plumbing involves work with a wide variety of materials, quite ancient tech but nonetheless fascinating.
Good news is a wrong choice makes little difference b/c if, after a few months of plumbing, you believe you chose wrong, you can easily train up in A/C/heating in less than a year.