I'm looking to add a #3650 Searchlight Car to my postwar set.

I recently purchased one for my son, for what I paid I should have got him the LIONEL TRAINS AEC SECURITY FLAT CAR No. 3535, as the light rotates and it is just a more fun car.  However it works fine and they are easy to find on eBay.

I picked up a 3520 at York a few years ago and it is a great addition to a postwar layout. It's fun to see the searchlight shine on the walls of my train room. The light is bright and the lens will rotate as long as you use a dry powder lube.

The Lionel 3520 has a vibro-rotating light, and a switch that can be remotely activated/deactivated with an uncoupling track.  The Lionel 3620 is basically the same without the remote activating switch.  The 3650 does not rotate at all, but can be removed from the car.  I guess it all depends on what you want to do with it.

I personally like the 3520, but you need to be sure as with any lighted car, that the 3rd rail contact roller is clean, and has a decent spring to hold it down on the track as you roll around the layout.

I felt that the 3650 was boring, so it has been relegated to the garage archive box.

I picked up two 6520s last year off ebay both for $30 including shipping.  They needed a cleaning and the wire needed to be replaced.  They turn on an off with the uncoupling track.  Probably the 3520 would be the most fun.  My 6520s are on the track and now have a Milk car to join them. 

I will also recommend the 3520. You don't have to run your train at Mach 1 speed for the searchlight to rotate, and it can be turned on/off on an uncoupling track.

The 3650 is the searchlight extension car. As others have written, it is a relatively lackluster car. The searchlight is on a cable, and can be removed from the flatcar.
The car came with a black metal crank that is often missing. "Runner" grade cars can be had for as little as $25-30. It goes up from there.

The car only has a single pickup, so it will flicker going over switches and coupling tracks. 

Thanks for all the info!!!  Based on popular opinion looks like the 3520 will get more use, so 3520 it will be!!