Not sure if I would call rDUINO Scope an improvement over OnStep. It is a different project and each project seem to have some advantages over the other one.
The main features of both projects seem to be quite similar, both projects support the LX200 protocols, can do tracking and guiding, meridian flips, both connect via Bluetooth, both can be controlled via SkySafari/Stellarium...
Here are some differences that caught my eye:
rDUINO Scope:
- allows stand-alone operation (while OnStep needs a mobile phone or a PC to run).
- supports RTC, GPS, screen and joystick (OnStep uses the phone app for this functionality)
OnStep:
- can be controlled via WiFi/LAN and Bluetooth (rDUINO Scope supports only Bluetooth)
- can even be controlled via a web browser as it can run a web server
- also supports Alt/Az mounts (think Dobsonians) in addition to GEMs
- has PEC
- comes with a dedicated ASCOM driver and a dedicated Android control app.
- supports more stepper motor controllers (Big Easy Drivers, DRV8825, RAPS128, TMC2100 and TMC2130)
- can run on other platforms than Arduino (Teensy, Tiva Launchpad)
So, it will all depend on what you want to motorise and how you want to control the mount.
And let's not forget the
www.astroeq.co.uk.
Note that I may be biased as I did software development for part of the OnStep project a while ago (but I don't use OnStep any more)