Orion Nebula and Crab Nebula - One good and one not so good

[stevewanstall]

I have been using a 1 1/4 inch UHC filter but this had been causing bad vignetting that was difficult to remove during processing. So, I  left out the filter so there was a 'straight 2"' opening into the camera. However, the light pollution meant that 45 s was the maximum exposure. Anyway, I took 65 x 45s of Orion and 165 x 45 s of the Crab - one looks good (for me ) and one looks bad, even for me !  I tried the processing several times but this was the best I could do.

So, the advice I am after is, is the poor image of the crab due to the nature of the nebula? my camera is unmodded.