Both are exclusives, both have been played in a beta form, both are third person action games and both are amazing looking games. Why not to compare them, at least visually?  
Click on the pics to see them full size:  
 
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- The results:  
- Gears 3 vs Uncharted 3 Ground textures: Gears 3. - Gears 3 vs Uncharted 3 texture work in general: Gears 3 has better textures work. - Gears 3 vs Uncharted 3 lighting: Both games have great lighting effects. - Gears 3 vs Uncharted 3 motion blur: Gears 3.  
Remember that both games have been played only in beta form which means that both games might improve visually when both games arrive this year. It will be interesting to see how the final games will turn out to be.