You are getting better,i should say ^^
The first thing you need to work on is light.Taking the first signature you can see light on his helmet and his upper part of arm,but no light on the background or any other light sources.You should see where the light is comming from,especially if render was lightened to begin with. Use burn tool and burn darker parts of render to make light parts stand out more.
Of coures more blendning needed

C4D on first tag looks like they don't belong there.More adjustment layers,my friend.Don't be afraid to go crazy in that part.
So the next is text,i belive it's better to simplify text as much as you can cuz what you did in the first signature doesn't look very good.This rectangle line or shadow (not sure how to call it) goes through half of your tag,which i don't find appealing.
I'm totally confused over second signature,to be honest
The right part of of c4d is a no no,they ruin your concept.You did better on lightning here but again look at his face...there is a strong yellow light that basically comes from nowhere.Well,same thing over blending and depth but don't worry,it's metter of practice.
Ahh and about motion blur,bubble c4d and background has different vector directions.
Overall: i like that you attempting to do signatures,really.Avatars shows only 1/10 of total personal skill,when other piece of arts can show true potential of the artist.It's a good sign that you will improve and rock GFX section someday >.O
KIU
P.S sorry for my english >.>