An oil cooler cools the engine oil. A transmission cooler cools transmisson fluid. Most trans coolers are installed in front of the radiator and work in series with or in place of the existing coil in the radiator that heats/cools the transmission fluid.
(Reality check: If your car is running hotter than it should be, you will need to fix the cooling system first before adding additional coolers. A new radiator and/or water pump may be needed to bring the cooling system back to it'* original function.)
If your cooling system is okay and you just want to go cooler, then I would suggest a lower temperature drilled thermostat, and a transmission cooler. Tru-cool low pressure drop trans coolers sold at http://www.bulkpart.com
will do wonders to keep trans temperatures down and extend the life of the trans with the nifty side benefit of lowering your engine temperature significantly. These coolers use oil viscosity to prevent cooling of the trans fluid in winter weather, but let all the fluid flow through when it gets hot and needs to be cooled. No moving parts, dependable, reliable. I bought the $40 model - wish I had bought the $60 one. They come with everything you need to install them and good instructions.