This is just my opinion but…
In terms of development and future proofing, the OSPi is clearly advantaged. Active development is ongoing on different applications and plug-ins as you can see in this forum. Also, if you are capable you can contribute to the solution easily. One of the apps is developed in python, the other in C++. It is just easier to write code for than the Arduino (and I have done both.)
In terms of reliability, the advantage goes to the Arduino version. Nothing against the Pi, but a microcontroller solution is just more reliable in my experience. The OSPi has some unique issues with storage (you need a high quality SD card to handle lots of writes) and networking (that WiFi dongle is notoriously challenging.) If you want a rock-solid controller that you can set and forget, this is the solution for you.
The Arduino version draws less power. The Pi draws about 5W while the Arduino uses about 1.5W. I wouldn’t choose the Arduino because of this though. It feels like a minor consideration.
The type of intelligent watering you mention is being developed in the OSPi version. I don’t see this being worked on for the Arduino variant but I don’t follow that as closely.
Scott