i went into the dealer intending to buy the quilt expression and came home with the performance 5.0. i have been very happy with my decision.
my machine senses when i have the single needle plate in and yet it doesn't say that it has that feature on the website, very odd and i can't remember if the qe4.2 had that or not. also the performance 5.0 has the setting for the 6d, dynamic spring foot that is just wonderful for free motion work and yet i can't find it on the list. good thing the machines are way better than the website :-) if you like to play with different stitches the stitch creator is great fun and lets you make and save lots of variations of your stitches. a handy dealer and some lessons will really get you using that feature well.
once you get past the basics, everything on our machines is really just for fun so if you are trying to be super practical about it the 4.2 is a great machine. if you like to play, more toys are more fun and the performance covers the basics and the toys very well.