Ramban

FOR IT IS A STIFFNECKED PEOPLE. This is to be understood in its literal sense. G-d is to go in their midst because they are a stiffnecked people, for now that the Holy One, blessed be He, became reconciled with them, His Presence amongst those who are stiffnecked would be better than that of the angel. For He will want to increase their blessings more, since they are His people and His inheritance. And just as at the time of anger it was better for them that He send before them an angel, because they are a stiffnecked people, just as He said, lest I consume thee in the way, so at the time of good-will it is better for them that the Divine Glory go with them, because they are a stiffnecked people, and He would more readily show grace and mercy upon His servants. And G-d answered him that He would do so, that He would make the covenant and do marvels because of it, just as Moses had asked for, so that we are distinguished, I and Thy people., Verse 16. He said, before all thy people I will do marvels… and all the people amongst which thou art shall see, because all these great and tremendous things He would do with Moses and for his sake, and the people would merely be in the covenant. It is not possible, however, to explain that G-d was promising that He would now do with Israel wonders such as have not been wrought before and in all the earth, nor in any nation, for [we do not find] that after this [statement] there were any wonders done for them, greater than those which had been wrought for them at the beginning in Egypt and at the sea; on the contrary, at first there were wrought and done for them greater things. Rather, the purport thereof hints at the dwelling of the Divine Glory amongst them, and at G-d being with Moses, for splendor and for beauty, 28:2. in hidden and wondrous matters, as he said, ‘v’niphlinu’ (so that we are distinguished), Verse 16. [which is of the root peleh — wonder], and as I have explained. May the Holy One, blessed be He, show us wonders in His Torah.