When you say ‘giving me freedom to do everything’ do you mean hardware or software or both? If you are referring to hardware, I would say the standard assembled OpenSprinkler is a better choice because microncontroller pins are easier to work with when it comes to reading analog sensors, counting pulses (e.g. reading a flow sensor). The hardware includes some features that OSPi does not have, such as LCD, buttons, headers for radio frequency transmitter. Also, the assembled OpenSprinkler is an out-of-box working solution for a typical user, whereas OSPi is mainly targeted to RPi enthusiasts who have experience with RPi. Because RPi is not included in the kit, it requires some assembly steps and software setup (mainly burning an SD card).
If you are referring to software, the assembled OpenSprinkler is written in C++ (more specifically, Arduino programming language). I work closely with Samer to develop the firmware and app. We have a roadmap with planned new features and the primary goal is to make the controller as efficient and easy to use as possible. Modifying the code requires some experience with C++. For OSPi, the software is written in Python, and is user maintained. Currently the OSPi software is primarily written and managed by Dan. You will need some experience with RPi and Linux in general. The features are easier to expand if you are familiar with Python. For example, Dan’s software framework has a wealth of plugins and also has several forks where users experiment with new features.
So I would say if you want the most straightforward setup, or prefer to have LCD and buttons, you should go with the assembled OpenSprinkler. If you prefer modifying the code in Python, you should go with OSPi. Hope this helps you to select.