I've found that, for smaller projects that involve odd spaces and tough fabrics (which is what putting a patch on a pack is) that hand-sewing saves me a lot of heartache. It may not wind up looking quite as nice as a machine-sewn piece (at least, mine don't), but it's worth saving on the hassle of (possibly) having to seam rip and disassemble to get at the spot, figure out which tension is right for the multiple (thick!) layers of fabric, sew in an odd shape (circles are a pain for me), and then reassemble the seams.
Sit down with some nice music or a TV show that you don't really have to pay close attention to and get to it with a needle and thread. Won't take more than fifteen minutes or an half-hour if the patch is normal sized, even if you're as thumb-fingered as I am.