They both have the same features. Aperture was far better prior to Adobe's release of LR4, but now they are alike.
LR4 is better integrated with Photoshop, but printing books from Aperture is definitely a far greater experience.
I tried both but decided to stay with Aperture. Certainly I would never use both, because that would mean having referenced libraries and that is NOT a good choice for me.
Also, consider that Aperture is $80 while LR4 is $ 149 ( nearly twice the price for almost identical features !! )
Hope this helps.