D3X/D4S/D4 - Top notch product produced by Nikon. Seems this would be used by professional photographers making a living with their camera. $5000 plus.
D810/DF/D800- Excellent product for pro's and amateurs who just are not quite ready to, or just can't justify throwing huge bucks at camera at this time. $3000-3500
D750/D610- Fairly affordable camera and a great product. For someone building a business or who really enjoys photography as a main hobby.$2000-2500
D7100/D7000/D5300/D5200- A second tier DSLR for someone interested in photography, or the family photographer who would like to spend a little more then your entry level camera. 1000$ or less
D3300/D3200/D3100- Someone whose enjoying a DSLR camera for there household, or just breaking into photography. Their first DSLR....welcome to the addiction. $500
I think in your classification there should be a first level that defines Fx (full frame) and Dx (crop sensor) as different categories. Among crop sensor bodies, D7100 are top level cameras.
Among Fx, Df stands in a specific category, being an apparent copy of an analogic SLR and made for nostalgic photographers (could be me too in principle

).
Said that, if you are on a budget, and provided that your wife already has some lens, stay on crop (and there, D7100 /D5300 are already huge upgrades on D3100). If she only has the kit lens, again, almost anything will be a good upgrade. Maybe plus some good lens.
If your budget is flexible, for the kind of pictures she takes, yes, better to consider a full frame.