We have measured the electronic structure of the two-dimensional electron system (2DES) found at the Al/SrTiO3 (Al/STO) and LaAlO3/SrTiO3 (LAO/STO) interfaces by means of laser angle resolved photoemission spectroscopy, taking advantage of the large photoelectron escape depth at low photon energy to probe these buried interfaces. We demonstrate the possibility of tuning the electronic density in Al/STO by varying the Al layer thickness and show that the electronic structure evolution is well described by self-consistent tight binding supercell calculations, but differs qualitatively from a rigid band shift model. We show that both 2DES are strongly coupled to longitudinal optical phonons, in agreement with previous reports of a polaronic ground state in similar STO based 2DESs. Tuning the electronic density in Al/STO to match that of LAO/STO and comparing both systems, we estimate that the intrinsic LAO/STO 2DES has a bare band width of ~ 60 meV and a carrier density of ~ 6 10^13 cm-2.