With the 4-4-0, you're going to have to gut it, at least to some extent. All the wiring goes to a circuit board in the boiler (lights, motor, etc.) I believe this slides out the smokebox, so it's at least marginally accessible. At the very least, most all of the wires you need are located there, so you can tap into the lights, motor, track pickups, etc. from there and wire in your own board in its place. There are ancillary circuit boards for the firebox lights, and a filter on the motor which may or may not be compatible with your DCC. I know it's not with the RCS R/C, so separating the boiler from the chassis might be part of the process.
The C-16 has the advantage of having all the wires routed into the tender, where it ties into Aristo's version of the so-called "universal" R/C-DCC plug. If your flavor of DCC or R/C has a board that plugs directly into that, then you're in good shape. Otherwise, it's similar to the 4-4-0 in that all the wires are there, you just need to cut them and attach them to the appropriate spots on the new board.
I love both locos. The new C-16 drive is near bulletproof and quite smooth. It's a matter of aesthetics. If you're doing 1:20, then you'll find the C-16 far too small. If you're doing 1:22 or 1:24, then either loco will work well.