"going full frame" - all the cool kids are doing it. 17-55 provides a similar FOV on a crop body as the 24-70 on a FF. If it were me, I'd get a used 17-55 and then sell it IF the day came where I 'went full frame'. You won't take much of a hit, if at all, buying used.
50mm is 50mm no matter what "view" we have, but not a similar image. If you're shooting people its best to stay above 50mm unless they are silly skinny and you want them to pudge no matter crop job or not.
when i had my D7000 crop i shot a 24-70 and 90% of the time it was at the longer end. I shoot more people that landscape, and even landscape I like the compression of the longer lens over the distortion