First, I was wondering if there are other cold sleepers who have had warm nights with 20-25F? I tend to be a cold sleeper, I sleep in a windy tarptent, and I'm going to places where is can become down to 20F (only during night).

Besides I saw that the Helium and the Helium EQ have the same amount of down, but the EQ is considerably warmer, how come?

I've also been thinking of the Marmot Never Summer, but that one might be to warm since it can be up to 60-65F as well, where I'm going or the Marmot Sawtooth (which is of course way cheaper then Helium) which might be to cold...

Thank you very much

I think I would bet that the EQ is a touch warmer than the non-EQ version. The other answer is correct about the EQ just being a waterproof/resistant shell but this will also reduce wind blowing into the bag and also reduce breathability...both of which will likely make you feel a little warmer. However don't buy it just to be a litter warmer, spend the money on a 0 degree bag instead.

From looking at the two products (EQ and regular Helium), seems as though the EQ designation refers to the outer shell. The EQ uses Marmot's proprietary Membrain breathable water barrier while the regular Helium uses a Pertex shell. That's where the extra cost is incurred. The two should be equally warm. If you're a cold sleeper, you might want to get a 0 degree bag just to be sure. With down, you wouldn't be adding too much weight. Also, be advised that you could get the Helium and likely use it with no issues if you had a really good sleeping mat. It's all about insulation from the cold ground. That's where most heat is lost because the sleeping bag insulation underneath you doesn't do much when compressed. At the very least, you should be using a 4-season Thermarest (e.g., Prolite Plus) or one of Big Agnes down-filled mats. Also, look at Exped Downmats. They have varying levels of thickness (translate: warmth). They're flagship model is rated to -36. Yeah, probably overkill for you--but you get the point. It's all about the sleeping mat. A good sleeping mat will let you use a less-warm sleeping bag. Also, a thermaest/Z-rest combination is also quite popular in cold winter environments.