Depends on what you mean by best option
You can not solder directly to the motor and light wires in the tender. They are in the loco. You would have to put the decoder in the loco. Keep in mind, that the light function connections in the tender do not connect directly to the lights, they are simply on/off inputs to the real lighting control boards. At least some of these must be in the loco.
You can probably do a sort of plug and play installation by connecting the NCE decoder to the plug board in the tender and removing the dummy board. There are solder pads on the plug board.
Putting the decoder in the tender will only give you primitive control over some functions. At this time I don't see how any of the FX functions will work or any of the locomotion effects from Soundtraxx will work. All of this is just speculation since nothing has been published. Until Mine gets here, I can't say for sure how difficult a complete install will be. There is no word as yet if the smoke generator is able to be controlled from solder pads on the socket board. Most of the questions I have asked have been met with "a function decoder installed in the loco will have to be added."
It looks like connecting to the socket board only provides for only the very primitive installs where the motor runs and the headlights change or maybe some other lights might be turned on or off.
Of course a lot of the stuff is like the Shay, as TOC said, "Yank out board and toss it." I replied that: "I would keep the card in the Shay, It provides all the tie points for that mass of wires."
Naturally TOC was right. Most of the wires came off the card anyway, Had to rewire most stuff to get it to work, When I got ready to button it up, I realized the stupid card was in the way, and all it was doing was tieing the four wires from each truck together, And naturally, one of them had a bad solder connection, so I ended up tossing the card and just soldering the wires together and heat shrinking them. God I hope it's not that bad with the Kay. It would be really nice to be able to swap decoders from one loco to the next at times.
On the bright side, I've confirmed with Digitrax that the new SFX0416 sound decoder takes a 0 to +5 or higher signal to activate the chuff.
to quote from the Digitrax site:
And, this additional bit from AJ:
For the Digitrax sound CAM input signal, all that is required is a source of voltage that goes "on" (above about +5V) and then "off" (no voltage for e.g. 10mS). The actual DCC track side is not critical. The only issue would be for DC operations when the track connection in one direction makes the signal always negative and unreadable. For operation on DC then, the cam signal will need to have a +ve power supply level, as e.g the new Bachman K27 seems to have.
Woopeeeee! no transistor to wire in.
So far, it seems to be the only sound decoder that does not need a transistor. At least it is the only one any manufacturer will confirm for me.