Functionally there is not a massive difference. They are both hooded down jackets with identical insulation. The helium is Peak Performance's lightest most packable down jacket, where the helium has a wind and water resistant outer as well as two functional pockets on the chest and sleeve.

No it is not. A down jacket is not typically a waterproof jacket, in an ideal layering system you would pair this with a waterproof shell jacket over the down jacket. In this case, the down jacket provides the warmth and the waterproof shell keeps wind, rain etc. off the down layer. You can take this further by adding a fleece/midlayer under the down jacket, and a baselayer under that!

Polartec Power Grid is an ultralight fleece fabric developed by Polartec. It has a "grid" construction which has raised squares or channels of fleece which sit against the skin. The air pockets or channels between the "grid" allow excess heat or vapour to escape, which makes the fleece super breathable and prevents overheating. What this gives you is a fleece with high thermal efficiency (warmth without the weight), extreme brethable properties and a very compact pack size.