Merging multiple images

By accident I shot various images of SH2-112 and SH2-115:

Both nebulae:	SH2-112 only	SH2-115 only

I was wondering about ways to combine these:

1. Treating them as a mosaic, i.e. preprocess separately and then combine (using Average - not Overlay in GradientMergeMosaic!!!)
2. Preprocessing them combined

The first one was fairly straight-forward. I preprocessed them as I always do and then used the GradientMergeMosaic process in Pixinsight.

For the second one, I did the following:

1. weighted them together
2. calibrated them separately
3. registered them to one of the images that has both nebulae
4. stacked them together

Here is the outcome of both processes:

Mosaic	Preprocessed combined

Well, that's pretty clear ...

But I still wanted to find out if the stacking of all images improved the quality of the image. So, I cut out SH2-115 from both images and compared those:

SH2-15 in mosaic	SH2-115 in combined image

Interestingly enough, the mosaic image seems to have a slightly higher SNR. So, I'll go with that method.