Since you already have and like a Janome machine, I'd stick with that brand and just get one of the 3/4 size machines. I bought a Jem because the dealer told me how easily she can take it on plane trips (carry it in a cloth bag and stick it under the seat in front of you).
The featherweights sound nice if all you need is a straight stitch, but I wouldn't want a machine for class that didn't have zigzag capabilities (needed that for a basket-making class recently) or needle up/down.
I don't know the differences between the Jems and the other small Janomes, but I'd want to be sure whichever model I picked had the specific features I want/need. Also, I don't think that feet and accessories are always interchangeable among all the models. I know when I've looked on Janome's website for accessories <http://content.janome.com/index.cfm/Machines/Accessories> that some things have notes specifying which machines they fit.
Have fun trying out and choosing a machine,
Margaret