Spray on is VERY hard to use, it bubbles and pits, and can run. The glass has to be properly conditioned for it too. You really can't see the irregularities on the outside, but you CAN on the inside. It is also harder to get all four a uniform shade with spray on. Paint, you can put layers and layers, and it is still the same color. The spray tint, the shade gets darker and darker with each layer; you can black it out...
They also recommend you remove the glass, well, that's another job...
You can go to a place that will CUT the tint for your windows, for $200.00 or less, INSTALLED, or just buy the cut-outs and put them on yourself. It's harder the more work you have to do...