They are both the same size connector and can be used interchangeably however if you are pushing your 15 inch on a 65w charger it may well still have battery use as well. This can happen on the 15 inch with the 85w as well but you really have to be maxing out your system and a desktop may well be a much better option for you at that point.
T-shaped MagSafe 1 predates the L-shaped ones, most notably the white/black MacBooks (non Pros) used T-shape plastic, then later with unibody there was a period of L-shaped metal, which was more durable but less versatile since you can only use it one way without blocking ports. Then I think with MagSafe2 it has only ever been T-shaped metal.
With each MagSafe generation (or any other plugs for the matter), the individual wattage output rating of the brick itself is independent of the plug shape. In practice you can use a 85/87w brick meant for 15" MBP with a 13" or MBA as long as they share the same generation of plug, since they require less watt. And vice versa is possible as well, just with slower charging or in some cases drainage faster than charge if the machine is actively used.