It’s impossible to be certain, since we don’t know the detailed mechanisms of two pieces of magic that are involved (each of which is highly non-trivial on its own).

My guess was that Voldemort would still be able to leave his body as long as the Process of the Timeless has not quite snapped into place (which did not happened until a few moments after Harry lost the cloak.) Leaving his current body voluntarily might take some time, if the descriptions of the Sphere-of-Stars-spell is any indication, so he might not be able to finish that process in time. (Killing his body, on the other hand, would presumably expel his spirit instantly, so he could escape in that manner.)

(… all of which is, of course, just a fancy way of shrugging my shoulders and saying “I dunno” ;) )