|SN2011fe in Messier 101, the Pinwheel Galaxy.|
|SN2011dh in Messier 51, the Whirlpool Galaxy.|
It would appear from a comparison of the pictures that SN2011fe is a bit brighter than SN2011dh. This may have something to do with the fact that the Pinwheel Galaxy is a little closer to us (21 million light-years instead of 24), but the imprecision in our knowledge of the distance to the Whirlpool Galaxy (plus or minus 3 million light years) means they could actually be at very similar distances to us. It's likely that SN2011fe is intrinsically more luminous than SN2011dh, and that has to do with the fact that they are two distinctly different types of supernovae.
SN2011dh was a type IIa supernova, which means it's the last dying gasp from a star several times the sun's mass. In contrast, SN2011 was a type Ia supernova, which means it came from a white dwarf in a binary system, with a mass known almost exactly: about 1.4 times the mass of the sun. The reason for the vast difference in energy output is because in a type IIa typically only a small percentage of the star's mass is ejected as debris, perhaps a few solar masses' worth. The remaining material is left behind as a neutron star, or, in some cases, a black hole. In contrast, a white dwarf undergoing a type Ia supernova explodes, completely, utterly, and finally. There is no stellar remnant left behind after one of these goes off. The uniformity in their masses before explosion make supernovae Ia valuable as standard candles in the universe for estimating distances to far away objects, and the fact that we now have one to study right in our backyard, as it were, is a great opportunity for astronomers.