If it were me, I'd certainly prefer to deinterlace and downsize 1080i50 to 720p50, than convert 1080i60 to 720p50. I don't know what's available on a Mac, but with a PC I know I can do the 1080i50 to 720p50 conversion pretty nicely (at least with most any typical footage) without a whole lot of hassle. Taking 1080p60 (or 720p60 to 720p50 though, would require either simple (crude) frame decimation (which can make for less than ideal judder - little jumps every 5th frame - ok for some footage I suppose, but not so great for other types of footage, like slow panning for example) or getting a heck of a lot more involved with frame interpolation (like with something akin to Magic Bullet or something like that).