Take your time (don't send anything to UCAS until April at the earliest!).
Of the two Aston has more *experience* with offering placements - so when you visit both Kent and Aston I'd suggest you ask a lot of questions about how the placement gets organised, where you'll work, what pay you'll get, how much you'll have to do to sort out the placement or will they support you, what the process is like for when you come back in your fourth year (at Aston almost all your mates from 1st/2nd year will also be coming back - will that also be the case at Kent or will you be bundled back in with people from the year below you?), what the deal is for accommodation for the final year etc etc etc.