I have used both the RUMBA and (currently) the Smoothie.
Smoothie Pros:
* Firmware current control of steppers (this actually makes them more difficult to tune by ear; I like using the trimpot!)
* Ethernet interface (just a serial-over-ip interface; the web thing is no replacement for octoprint)
* Configuration is flexible and clear, this and firmware itself (!!) can be updated on the fly via USB/SD card. No arduino IDE / flashing procedure (this is tops)
* Nearly every control pin is broken out to a header
* SD card is provided/integrated/necessary, works straight out of the box (supposedly the most reliable way to print from this board)
* Extra grunt reduces chances of non-time-critical things like LCD getting in the way of stepper/PID control (these issues pop up every now and then on 8-bit boards).
Rumba Pros:
* More logical board layout (slightly, definitely a subjective opinion!)
* Removable pseudo-standard driver modules
* Cheaper (just)
* Marlin firmware is more 3d-printer-specific, and development is livelier, so new/fun features likely to be supported earlier
In summary:
I believe the advantages of the smoothie's performance (32-bit 96MHz ARM) currently do not translate into real value gain for a 3d printer - even though the extra grunt could in theory support better temperature, motion and extrusion control than an 8-bit uC could (16MHz 8-bit AVR), which was my main hope in trying it out, in practice the firmware basically implements the core functions similarly to Marlin (GRBL-based motion control, vanilla PID for temperature control, no (working) extrusion advance). The only present advantage is the ability to run higher combined steprates (e.g. for running a couple of 256x ustep external drivers, for example) - which is usually a little outside typical needs.
Get whichever you can afford and like the look of, as either will likely serve you well. But hoping that the smoothie firmware starts taking advantage of the hardware is a gamble that has kept a fair few people waiting for a couple of years already!
SD3. Mk2b + glass, heated enclosure, GT2 belts, direct drive y shaft, linear bearings, bowden-feed E3D v5 w/ 0.9° stepper
Smoothieboard via Octoprint on RPi