thats right, they normally come pre made with a resistor in, easier to get in but the shape can hinder sometimes you have to look at what needs you have, use a resistor when you need lots of lamps for the effects, and the 12v for lack of space and options.
you can get holders for all sizes of standard leds, makes it easier to mount space permitting. I fitted a couple of 10mm yellow leds on my door mirrors cause i was desperate to get my rabbits on the car somewhere, so took the side indicators off the wings onto the mirrors, naff some may think but its my car!!
