Most of the dedicated hawk counters I know use 8X bins. Some use 10X but most default to 8X because the magnification is, generally speaking, more relaxing. Also, 8X usually has a greater FOV. I've used 7X, 8X and 10X. Since 2004, my 8X32 SE has seen the most hours but this year I suspect that will change. The 10X50 Swarovision is so much fun I plan on using it almost exclusively, especially for raptors.
The question is really based on duration. If you're staring at the sky through a hand-held bin for long periods of time your physical strength is the determining factor in your ability to locate and identify birds. A shaky bin guarantees you'll miss distant birds others are talking about. All things considered lower power = greater stability. A 10X sounds great at the keyboard but if you can't hold it steady at the hawk watch you're not going to find much in the sky.
Believe it or not, the past 2 seasons I've been using my old monarchs which are 12x42! Some days were upwards of 8hrs. :eek!: