In fresh powder, unless it's only a few inches of it, you won't get much glide on any skis. Wider will float you more, so you'll plow less, but you won't get much more kick and glide because now you have more surface area and thus more friction. I suspect you'd notice the downhill difference more than on the flats - wider skis are meant for downhill skiing in softer snow. As soon as the snow firms up, you will go faster and farther on your current skis than you would on wider skis. Longer skis will glide better than wider - going longer at the same width gets you more flotation without sacrificing as much glide, as long as the skis are the right length for your weight. If you are too light for the skis you won't have good grip; if you are too heavy for the skis you won't have good glide. Which length is right for your weight is different for each ski.
Your BC 70's should be a pretty versatile ski - just narrow enough so you can use them in prepared tracks, and not too skinny for off-track use. If what you are doing is poking around the woods in easy terrain, I'd stick with those.