I have the standard Helium and Lithium, and had (sold) the EQ.
There was a slight loft difference between the standard and EQ heliums, which I assume was due to the heavier fabric. Note that I do mean slight, perhaps between 1/4 and 1/2 of an inch… but noticable if you were looking for it.
I used the EQ for two weeks on the Long Trail a few years ago, back when I still did such things. Although I was never cold, and the bag packed well, after a few days on the trail, I noticed that it started to lose its re-lofting ability. My habit was to get to camp (shelter or tarp) and immediately unpack and fluff up my bag to allow maximum relofting and facilitate drying. Despite this, the bag appeared to lose about an inch of loft over a full week out. I have not experienced this with the standard Helium, and can only assume that the EQ fabric does not breathe well. Once I could let the back air out for a full day, it did return to original loft however.
Keep in mind that thsi was October in Vermont- pretty humid, nightime was between 30 and 40.
If you intend to use the EQ as a winter bag, with constant temperatures below freezing, I'd get a VBL system. However, the water-resitant shell would be great for frost and the freezing of your breath around the hood opening. More than once my frozen breath has coated the outher of the Quantum on my Helium and Lithium, and either saturated the fabric or was pressed into the bag due to movement- even while attempting to brush it off. So, for winter, the EQ might be a good option, or you can get an ultralight bivy for the standard. I use the Equinox- light, cheap and effective. Regardless, for winter, the Heliums are not warm enough for me- even the Lithium almost let me down in single digits a few months ago (yeah, I sleep cold).
The full zipper has never been something that has been either a positive or negative attribute for me. I have no problem using my standard Helium as a quilt, but I also rarely sleep out when the temperatures are above 50 degrees.
The perfect bag for me would have the EQ fabric around the face opening and on the inside of the hood, and Quantum everywhere else- RAB does this, but no one else I can find.
EDIT: Just found the weights (both in regular size)
Helium- 30.8oz
Helium EQ- 37.1oz.
This was measured on a Pelouze digital scale. As you can see, adding a 6 ounce bivy makes the two systems the same weight.
