If you are good at soldering and want to build one completely from scratch, go with 1.42u. Yes, you will be locked into 1.8.3 firmware, and there is no way to add SD card support due to the flash and RAM constraints. I wish Atmel could release an ATmega648 chip at some point, which would have solved this problem 🙂
If you are familiar with Python and want to tinker with the software, go with OSPi. It currently runs firmware 1.8.3 (Dan’s interval_program). There are also Rich’s sprinklers_pi program, and the Google Calendar based scheduling program.
If you want something that works out of the box without further assembly, go with OpenSprinkler 2.0s. It’s more expensive, because it’s more oriented towards the consumer market. But it’s still open-source, and give you plenty of space for hardware / software hacking.
If you want something in between fully assembled 2.0s and the DIY 1.42u, there is the DIY 2.1u that will become available in a few weeks. It’s not all through-hole though — about 60% assembled and the rest for you to solder.