Canon Rebel XS, Canon Rebel T1i, Nikon D3000?

My dad got me a Canon Rebel XS for christmas. I'm 15 and am sort of an amateur photographer. I have had 3 cameras(all P&S). The first my sister broke when she yanked it out of my hands and dropped it. The second I still have but I wanted an upgrade so my mom let me get a new one last January. It is a Nikon S4000.It's a great little camera but it doesn't take action shots worth a darn. I'm an avid Equestrian rider(I ride horses if you don't know what equestrian means). Currently I'm training Hunter/ Jumper( Jumping my horse). I tried to take pics (well my mom took the pics but I had the camera stabilized and set so all she had to do was press the button) but they were always a blur either my horses legs or me and my horse as a whole just a giant blur. It also doesn't take pics at night or in low lighting. I have used my moms old film camera and I took some really nice dew drop pics with it. I got up early in the summer and headed out into the pasture, I also got pics of the horses laying down. I know how to treat a camera so please don't say get a cheap DSLR. My dad asked if this one had all the stuff I needed and if it didn't he would exchange it. It is a Canon Rebel XS Deluxe Kit. Does a Canon rebel take good action hots I know it takes 3 fps but is that good enough? I know mega-pixels don't matter unless you are going to print a pic and enlarger it but Should I go for one with a higher mega-pixels and fps?(it has 10.1) I think one of the cameras I could exchange it with is a Nikon D3000.It has 10 mega-pixels and 3 fps, as well as an ISO range of 100-1600. The Canon rebel XS has an ISO range of 100-1800. I think on other option is The Canon rebel T1i.

Added (1). I checked the specs for a bunch of different cameras awhile back. I'm on the Yearbook Staff at my high school and was looking in to getting a camera for that. My P&S didn't take very good pics at football games or in the gyms so I had to find out what feature was the lighting part and that was it. The Nikon d3000 also has 11 auto focus points and the XS has 7. Not sure about the T1i but I think it has 11.

The Rebel XS is similar to the Nikon D3000

The Rebel T1i is similar to the Nikon D3100, although the D3100 can auto-focus in the video mode.

To get good action shots, you do not need to shoot at continuous mode. Actually precisely timed shots will catch the action

Here is a sample of an action shot taken just at the time I wanted. NO continuous shooting was involved.

It just takes practice.

I would suggest you get the D3100 if you can. That camera has features that will keep you busy for over five years.

Assuming the information about the ISO of your camera choices are right, then you should go for the Canon Rebel XS. Since you are concerned about the shutter speed then having higher ISOs really help. (ISO is the camera sensitivity to light. The higher it goes, the brighter your pictures can go. Just in case you didn't know). However, if you can get the Canon Rebel T1i, then you should go with that.
Better lenses should also help you in the long run.