I've used the Duotone Echo 7m extensively, and an f-one swing 5m a few times, but not the original Duotone wing, so my experiences are limited due to this, however what I found is that the Echo is VERY stable and easy to use in it's top end (when powered up) as well as having good low end power. From reviews I've read and watched everyone says the canvas is much tighter in the Echo compared to the v1. I have used my 7m Echo in 6-19 knots, playing around with the length of the boom from min to max settings. Because it's so stiff, and because its got a rigid boom it's the best wing for pumping In my opinion. The boom length makes a slight difference in handling - when the boom is at max extension it's a bit easier to handle when maxed out, but the trailing edge gets a bit flappy as a result. This isn't a problem in terms of handling, it's just that I noticed the noise. Of course this just could have been because I was very overpowered at that point - a 7m isn't designed to be used in 16-19 knots, (I would normally be on my 5m at this wind range) but I wanted to test it to see what it could do and what it felt like. I would say the 7m Echo is perfect for me (85kg, 6'2" intermediate skill) in 8-15 knots. It will keep me going in lulls down to 6 knots easy, and I can just about handle it up to 18 knots, but when consistently above 14 knots I would always swap to my 5m as over 15 knots the 7m is just too much of a beast and tires you out quickly.
The little experience I've had with the f-one swing is that it has better low end performance than the Echo, but I really felt the gusts which didn't lead to a smooth ride when in its upper power range. Both wings felt really easy to use and handled very well. It's a shame I haven't used the Duotone v1 so I can't compare it to the Echo.
The blue line will depower the wing slightly when you pull on it but it's not comfortable to do this unless you put a plastic tube over the rear section (eg from an old harness line) to protect your fingers when pulling on it.
Hope this helps.