The second one is of course better. Although getting a great GMAT is
an awesome way to show the AdCom that you are now a serious student it
still doesn't "make up" for a low GPA. And a student that has an
excellent GPA + excellent GMAT will be a priori a stronger candidate.
This does not mean you can't make it into a great program! You can! But
if you have as I understand a 730 - you do not need to take the GMAT
again. In fact it would be a total waste of your time. A 730 is totally
awesome! and already way over the average of all schools even HBS and
Stanford!!!!!
I find that there is the impression often amongst Chinese candidates
that if you only get xxx points higher on the GMAT you will for sure
get in. But even if the GMAT is very important, your candidacy will be
considered as a whole. And the difference between a 730 and 750 is less
important than anything else you can do. So do yourself a favor and
don't take the GMAT again. Instead use your time to research the
school, to build contacts, to get a bit more leadership experience. all
of these things will be more valuable to you in the long-term.