fab, i've been stuck between the D3 and D3s and need a camera that will get the sharp, crisp images.
If you aren't getting sharp crisp images with your current setup, you need to look closer at your technique.
Virtually any modern dslr will produce good images (and the 70-300 is a fine lens)
The difference between a $5,000 flagship Nikon and a $600 entry-level Nikon is pretty significant. That's why the professionals at Sports Illustrated shoot $5,000, flagship-level, full-frame, professional cameras, not cheap beginner Nikons and Canon Rebels. That's why the local newspaper part-time pros in my podunk small town shoot D3s and Canon 1D bodies...
The D3 is an okay camera, and in smaller USA markets at WALK-IN retail, D3 bodies are down to $1700 or so...I am not talking about on-line prices at
KEH.com or
B&H Photo, I mean in real, small- and mid-sized cities across the USA, a D3 is a $1700 body...
The D3s has a newer, better sensor.
The idea that a D5100 is the equal to a D3 or D3s when shooting sports or equine action is mis-informed. It is MUCH easier to shoot with a full-frame professional level Nikon than it is a beginner-level crop-body that has a slow mirror, a weak AF system, and a crappy, tiny,low magnification pentamirror finder. I've owned the D1, D1h, D2x, and D3x Nikons, as well as several lower-spec Canon Nikon and Fuji d-slr cameras; there is a very simple reason the D3 and D3s exist. Either one is streets ahead of the D5100. The main advantage is that the D3s has a noticeably better sensor at elevated ISO levels. The pixels are HUGE, and the per-pixel image quality of the D3s is excellent.
Being able to SEE the shot, and then hit the release and GET the shot, in-focus, is one of the main pluses of the professional-grade Nikon bodies. A D3 is a nice body. A D3s is the same body mostly, but with a better sensor. Buy whatever you can afford. Either is miles ahead of the D5100 in almost every,single metric that really matters in action sports. Of course, the photographer does have to have at least SOME basics checked off, but the body really does do most of the "shooting". It is the focuser, it is the shutter, it is the "film advance", it is the buffer, it is the "film"...the camera these days is much,much more than just a light-tight box. The user needs to point the camera in the right direction. Can you point the camera well enough to make use of the D3 or D3s? only one way to find out I suppose.