The Arduino can do nearly anything you want, and LED display is pretty easy. You can even make your own special fonts.
The Cubloc also is very capable, and it has a combination of Ladder Logic, and Basic, which can do lots. The Cubloc has a dedicated port for the keypad, which is basically "plug and play", and also a huge number of I/O. However, the keypad outputs an integer for each key, which requires a brain to do the action. It will also run on 24 volts, since it is really built for commercial applications.
I messed with the Arduino a couple of years back, and remember having difficulty getting a flat panel matrix keypad to work, but the Pokeys worked with the same matrix keypad without a problem. Probably something I did not understand going on regarding the Mega.
Pokeys also has a dedicated I/O for various keypads, and also can output to an LED display. Since the Pokeys has internal software to output Mach3 OEM codes, it is simpler to get things to work right without brains.
Using a Mega will require Modbus and brains to do the same things. They all have their strong points.
The Mega probably has the most digital I/O pins, and has 15 analog I/O. And, the Arduino forum has some really talented members; a lot of the discussion goes way over my head, although they will always chip in to help us "ordinary" guys.
Is it a basic Jog/rotary encoder Pendant you want to build, or something else?
John