Which Decoder Should I Pick?

[tompm]

Now that I have done some research thanks to Charlie and Dan a couple of questions have arisen.

With the decoders I have looked at have 9 pin connectors and the locomotives have 21 pin. Does that mean I have to get an adapter?

I noticed on some of the decoders a resistor is recommended. Does this refer to the installed headlights or are they talking about additional light features?