In my experience it's better to decide which features you *really* want to have, then test drive machines that have those features. It's no use to have a lot of fancy features you won't actually ever use. Even my vintage Bernina 1230 has features that I don't use. What's important to me is that the features I use on a daily basis work really, really well for me -- consistent and good.
The other thing to have solidly in mind at the beginning is your budget.
Both of the above will narrow down the field enough so that you have a manageable number of things to compare.