M27 - battle with DS imaging, some help needed

[riklaunim]

After some fun with mount alignment, setup optimisations etc. I've decided to make M27 in LRGB with UHC-S stacked on each channel. When I finaly was able to take those images.... some small fluffy clouds come and covered the star in OAG:

Click for 100% size/crop:

(1 frame, 300 sec, 83% gain, -dark, UHC-S + L, CCD temperature - about +30 C )

So I had to wait and when the clouds went... roof come into the FOV

Those images were 5 min exposures in bin 1, 83% gain for DSI III Pro in SCT 8" reduced to f/6.3 (1260 mm focal length). Single frames look very nice and a stack of 5-10 should give nice image. But I wonder - what would be better - more frames on shorter exposures, or less frames on longer exposures? And how gain fits in that exposure times (small gain on long exposures?)

To test some more I've made a lot of 15 sec exposures in bin 2 when the clouds were flying. I was able to stack 80 out of ~120 and got this:

And the same upscaled 50% by drizzle stacking: