Having been there and done that, I would suggest an upright bike with as low a step through height as you can get. It's the mounting and dismounting that is the hard part for either exercise bike design. The Recumbent design is just too low when you first start out post-op, and will require you to lift your leg in a most uncomfortable way.

Right after your surgery dislocation is your biggest concern until your muscles that have been cut and stretched get toned back up (they are the only thing that holds anyones hips together). Your Doctor will most likely give you a list of exercises to do both pre-op and post-op. I would consult your doctor before you add an exercise bicycle to your recovery. Good luck with your surgery... it will seem like hell for a few weeks, but then the pain relief is worth it.