There's nothing wrong with the Snugpak softies, other than they're expensive. Some of their other cheaper bags must be naff though if mine was anything to go by. It came apart at the seams in several places after just one trip. Was really disappointed in that bag it was way too bulky as well. I sent it back and it was replaced, eventually, although I didn't want it replaced I wanted a refund so I bunged it on eBay.
Down bags are overrated IMO. If you're camping in the cold and damp you're better off with a synthetic. Down soaks up all the moisture in the air + your perspiration and is then less warm than a synthetic. You don't end up looking like a chicken from all the down that escapes and sticks to your fleece either. Synthetic bags are easy to wash too.
I find it very strange that nobody has either one of those tents on here. Remind me what forum this is? I may as well have posted this on a gardening forum I probably would have had more response there might be room in it's porch for growing cabbages. I put that down to kit snobbery because it's a Vango. It can't just be down to price either because the Zephros tents are very popular. Very odd.
If anyone has got either of these I'd be interested on your opinions, and if used in anger in some Scottish weather all the better. The only thing that's off putting with the Helix is it's seemingly low height. I don't like sitting hunched with my head pressed up against the inner same as in my Zephyros (I'm about 5' 10"). Whereas the Zenith is a bit higher (also the heavier of the two). Both of these designs have their advantages & disadvantages.
The main thing I want to know is whether there's enough room for cooking. The Helix doesn't seem to have a porch as such but I assume the inner at the front can be unpegged and pulled back to make enough room for cooking? I'm not sure there's enough space in the Zenith without doing the same, more difficulty in pulling the inner front/side back with that design but that's what I do with the Zephyros but it is a bit of a pain.