I have both the Pipedream 400 and the Cumulus Ultralite 350. The Pipedream is a bigger bag, has better loft, neck baffle, so for me, is a better bag. The Cumulus has a noticeably smaller foot plug. The one advantage of the Cumulus is that it packs smaller.

The Cumulus is not a bad bag and the trapezoidal baffles are very effective, it's just the Alpkit Pipedream is an outstanding bag. 

  Personally, I would avoid bags without zips as I like to have some temperature control. I have a WM HighLite which has a half zip and even that is limiting. Full zips are essential for me, but others have different views and that's up to them.

Thanks for the input Robin especially as you have two of the mentioned bags.

It's very interesting you say the pipedream 400 is bigger than the cumulus U/L 350 as alpkit say the pipe dream is 210 x 70 and cumulus say the U/L 350 is 217 x 75cm.