The 10v and 9 are very similar, except that the 10v has a normal 2.5" SATA drive for storage, while the 9 has a strange SSD
card. Other traits are nearly the same: CPU, graphics, screen resolution (1024x600), etc.
The 10v does have an optional faster CPU, but it's barely faster. Also, early 10v's came with a 1024x576 screen.
The 1012 is similar to the 10v. Slightly better IO performance (only applies to fast SSDs), slightly faster CPU and graphics, same screen (1024x600), etc. The big difference is in the options, the 1012 has the option of a higher res screen (1366x768), video decoder card, etc). The 1012 also supports 64bit, but that won't matter to most people.
The 1018 is essentially a 1012 upgraded to use DDR3 RAM, and missing a couple features due to cost-cutting measures. No mic input, one missing USB port, few premium options.
The 9 and 10v are great netbooks, but being older means they are more likely to have older, beatup batteries with reduced capacity. I'd probably go for a newer model 1012 or 1018, unless you get a fantastic deal on a 9 or 10v, or you want a 9/10v for OSX.
Also, all the inputs and outputs are virtually identical between these models. They all have USB (x3), a card reader, VGA out, mic in, headphone out, 100Mbps Ethernet. (As mentioned above, the 1018 is missing a couple). I think they all even use compatible power supplies.