I have owned the Eton 300 Mini for more than 5 years now and because is was getting a little dog-eared, decided to buy the M400 to replace it.
However . . . In comparison, I find the following: In the looks department, the new M400 slightly has the edge, but I doubt the matt black finish though very smart will prove to be as durable as the 300 Mini. The sound quality of the M400 is well down on that of the 300 and is somewhat tinny. The telescopic aerial on the new model is much shorter and worryingly thinner - I don't know if this has any affect on the sensitivity of reception. Where it scores badly with me is that the same shortwave coverage that is achieved in 7 switched bands with the earlier model is now compressed into just two, making accurate tuning particularly difficult. Locating a station using the digital display is not easy, as the amount of movement of the tuning dial that is required just to "trim" the frequency spot on, is so small that you tend to overshoot, go back, overshoot - and so on. A larger diameter tuning wheel might help in this respect - but No, they've made it smaller!. I mostly use an indoor long-wire aerial for short-wave reception, as without it all but the strongest stations are too faint. In the name of keeping the thickness down, the new model uses 2 x AAA batteries and this reduces the battery life below that of the 2 x AA batteries for the 300 Mini.
I have to admit that my overall impression is one of disappointment, as Eton don't appear to have built upon the strengths of their earlier radio - rather they seem to have started with the design work of the quite impressive case and then perhaps been somewhat thwarted with regard to the choice of components that would fit inside it. Back to the drawing board with this one, I think.