Greg,
I agree with pretty much everything Kevin said, but there is one factor not mentioned. FX lenses will work fine on either a crop sensor camera (DX) or a full frame camera (FX). The reverse is not true. i.e. a DX lens on an FX camera. Your daughter may have a DX camera now, but could well have an FX body in the future. The problem is the image circle projected by the DX lens. It is designed to fit the smaller sensor in those cameras, and will not cover the full sensor in an FX camera. The corners will vignette. With a zoom lens, you can use a DX lens on an FX camera, but you will be limited in the zoom range you can use because of the vignetting problem.
In the digital world, camera bodies are getting better all the time. What is OK today may be really obsolete in a few years. Hard to say where the DX body cameras will land, if they have a future. Lenses don't change very much, and you need to make more long range decisions buying lenses. If you can say you will never have an FX camera, then the DX lenses are fine. I do own one DX lens (12-24mm). All the rest are FX. I would not have bought the 12-24 if I had forseen getting an FX camera body. I bought the 70-300mm for my wife to use on a D7100 since she just couldn't manage my heavier FX bodies and lenses while shooting birds and wildlife. Shooting birds and aircraft, the 450mm effective crop is a big advantage.
Autofocus speed and accuracy is mostly determined by the maximum f-stop of the lens and the autofocus system in the individual camera body. The lenses being discussed all have a max f-stop of +/- f/5.6 at the long end of the zoom range, so that is pretty much a wash. The ability of the autofocus systems in Nikon cameras has improved considerably over time, and the latest bodies will even autofocus at F/8. The previous limit was f/5.6. The faster the lens, the better the response of the autofocus.
The build quality on some of the prosumer lenses does vary, and some long zoom range lenses I have looked at appear somewhat fragile, as Ryan experienced.
Changing lenses can let in some dust, but reasonable care and technique will minimize the amount that lands on the sensor.
Edited 1 time(s). Last edit at 11/20/2015 05:08PM by jgunning.
