Two things I do not have an wish I did are differential feed and a free arm. I make bibs with a ribbed neck and used to make a lot of tee shirts --- therefore the free arm would be wonderful.
Speaking of "ease of threading" -- sergers can be difficult to thread - but I hardly ever thread mine because I cut off the thread at the spools that are on it and tie on the new color from there - run it through until your new color is coming through on all 4 threads and you're good to go.