I have the Echo cs-370 and Stihl ms-250. My uncle has the Echo cs-400.
370 is 16", semi chisel, safety, 3/8" LP
400 is 18", semi chisel, safety, 3/8" LP
stihl is 18", full chisel, non safety, .325"
All have the needles adjusted properly. None have any intake/exhaust mods.
Stihl is much faster (even when running Stihl RMC chain)
The 370 is only a little lighter than the Stihl when held out by one hand in a tree (Suprising since the Stihl bar is longer, thicker, and taller due to the larger sprocket nose). The stihl seems to have a little shorter of a body. Maybe that's why.
The Echos are much quieter. They also have adjustable oilers which I like, but I have never had a lack of oil with the stihl.
I usually pick up the Stihl first. Even on the ground I grab the Stihl over the Husky 455. I love it for it's weight, price, and power.
If buying new, I would pick the Stihl. I don't think I would sell the Echo to buy the Stihl though.
I have the Echo because it was a great price as a refurbished unit. Previous owner said the chain did not stay on. Seeing the chain that was removed I realized the previous owner had no idea that rocks dull a chain and therefore probably had no idea how to tension the chain correctly.