Electric rice cookers generally fall into 2 categories:

1) old-style

2) new-style "fuzzy logic" teflon coated.

The old style cookers are the inexpensive at any size and the rice cooked in it will have a firm crust where it touches the insert as it cooks. There are many tasty dishes that can be made from this crust but if you're not after the crust, then washing-up is a pain and there is wastage of the rice that forms the crust.

The second type is much more expensive and ranges from the basic model which is very similar to the old style cooker above (except that it doesn't produce the crust because the insert is teflon-coated) to the ones that have computers, timers and do everything except make a latte and tuck you in bed.

If you're only after rice and not the crust, and want fast and easy clean up then the new-style is much better.

Both the old and the new styles make good porridge, however if you also like to steam a dish on top of the rice as it cooks, you'll need to read the instruction manuals of the new style quite carefully. Some of the new styles require more complicated settings, special lids and such to do this. The old style does this with no problem. In both cases, the rice cooker should be slightly larger to make room for the dish on top.

I have the smallest basic Zojirushi "fuzzy logic" for 15 years and it is still going strong. It is perfect for rice for 2 (with leftovers for lunch) or rice for 1 plus a steamed dish (with leftovers for the next day). I should really be getting the next size up as my SO is finding out that he really likes rice now that he has discovered how easy it is to make and clean.

I also have an old-style cooker I reserve for making dishes that require the crust.

Just for reference, my grandparents have rice cookers in 2 sizes, one for just themselves and a massive one for when there's company.