I did a comparison when 14.04 was first released on the memory usage of different Ubuntu flavors. Some takeaways:
- Lubuntu has zRam (automatically compress memory to save space) enabled by default making it hands down the most usable version with low-memory. It's the only flavor to have it enabled on the LiveCD.
- The real cost of 64 vs 32 bit is usually only 128 MB.
- Lubuntu (32-bit) still boots with just 160 MB of RAM!
- The Ubuntu kernel can't boot with only 128 MB of RAM.
Raw testing notes