Which Software?

The older generation of AIBO runs on an external Memory Stick media with software installed. Each software has it's own quirks, which can change how the dog behaves. Each software will have a summary of how it works to help you decide which is best for you and your pup.
Do note I don't cover softwares like Party Mascot or Card Dog, which work better as extra softwares instead of a main personality.
This does not apply for the ERS-1000, which runs on a single software programmed into the dog.

ERS-110 & 111

Hello AIBO! - A trimmed version of the default AIBOware. They run on one default personality instead of evolving. Has a few skits not in AIBOware.
AIBOware - Raise AIBO from a baby, and watch it grow over time! This software evolves as you spend time with it.

ERS-210 & 220

Hello AIBO! - A trimmed version of AIBO Life. They run on one default personality instead of evolving. Has a few skits not in Life.
AIBO Life (1 & 2) - Raise AIBO from a baby, and watch it grow over time! This software evolves as you spend time with it. Both Life softwares run on both models (though 2 is designed for the 220).
Recognition/Orikou AIBO - A 210-only software that talks back! Known for having a rather grating voice and being extremely repetitive, but still a good friend. Most Recognition users prefer the Japanese version.
Explorer - Designed for the 220, but will also run on a 210. Behaves similarly to Hello AIBO! - but this time with Boost Mode and new skits/sounds.

ERS-311, 312, & 31L

AIBO Life (3) - Raise AIBO from a baby, and watch it grow over time! This software evolves as you spend time with it.
Pal (SE)/Friend - A trimmed version of AIBO Life. Pal is the English name, Friend is the Japanese name - they're the same software. Pal is known for being hard to keep happy, which Pal SE fixes.
Kawaii - Similar to Pal/Friend in the sense that it doesn't evolve. Has many cute and unique skits such as doing a headstand, but they're very harsh on the dog's legs and typically result in the legs becoming very noisy or slipping if not repaired.
Handy Viewer compatible softwares can be found under the 31X Series "Handy Viewer" page.

ERS-7

Mind - The first software released with the initial run of 7s. Speaks tonally unlike Mind 3, but still very loving and has fantastic sound design. Also referred to as Mind 1 for continuity with the other two Mind softwares.
Mind 2 - An expansion of Mind 1. Still doesn't talk, but can do a lot more in comparison to Mind 1.
Mind 3 - AIBO talks back! They'll ask about your day, ask to play, among other things. Still dog-like, but can operate like an assistant if you want them to. Their voice can be changed by editing their Custom Data on a computer. Generally seen as the best software for a 7, but is much larger than the other two Minds, needing a 128MB stick.