The dart is the commercial vac and the Felix is the residential model.
The Felix is a similar vac to the dart 1,except it has few more features such as speed control switch,adjustable handle and more accesories ( brushes,paraqat head etc)
Both a very good vac.
I have a couple of dart 2 that I have had for best part of 2 years and have tried or owned nearly every type of vac out there.
If its price go for the felix,if you can afford more go for the dart2 as it has a bigger brush head and stronger brush motor,which means you cover a bigger area quicker.