The main benefit of the Euston is that its rapidfire in an AR. I haven't tried the Rhett yet though. I actually have done pretty well with a Drognav and a Carb. The Carb is my favorite SMG right now due to its stopping power. Throw in a drum clip and you can deluge the area in fire, great for lights too. Of course, the heavy can carry a MG and a AR...
The Rhett's main advantage is that it is very stable. You can just hold down the button and you will have a very small cone of fire. You can just fire on full auto with a high degree of accuracy. However, it also has a small ammo count and doesn't do much damage per bullet in comparison to the Euston so you tend to run out of ammo quickly. The Euston has higher damage, but more recoil. It is also has good ammo capacity. Hence, the Euston has more potential, but takes more skill to handle.
The Rockstedi is nice. It's a shame I play in Singleplayer mostly, I die so damn much I can't get much fun in unless I play with ridiculously powerful weapons.