I'm looking for a slr camera that takes sharp pictures!!

Either of these camera's will be great!  However, the D90 is an older model, I'd recommend you compare the T3i to the Nikon D5100 or D7000.

Here are a few tips to get sharp photographs:

• Buy good lenses, I'd recommend getting a 50mm prime lens to start, prime lenses are usually very sharp, and they are great for taking natural light photographs and blurring the backgrounds
• Make sure your photos are in focus (learn the camera's focus settings and modes inside and out)
• Sharpen your photos in post processing, using software like Photoshop

If you want sharp pictures, you should focus on good lenses. Any recent DSLR with a good lens will do the job. For sharpness, primes are best, not zoom lenses. So, you could do as WaterlooAlex suggested and get the 50mm prime lens. As far as the body goes, you could take a look at Pentax bodies, the K-30 and K-5 particularly. They probably have the best sensors in its class, and the IQ is just stunning. There's also the K-5 IIs that's just been announced. It has no AA filter, which means your photos will be even sharper.

If your looking for a camera that takes great pictures and affordable.

Look no father then the Pentax k-30