I am going to buy a 130 or 250 soon myself, maybe tomorrow. The FS100RX won't accept any other attachments if that matters to you.
I have the FS90R. It is a great trimmer, but yes, they are a bit hard to feather smoothly. They run great at idle to about 1/4 throttle, then they tend to jump up to around 3/4 throttle or slightly more. I have no idea why they do it, or why Stihl doesn't correct it because their 4 mix blowers are extremely smooth throughout their throttle range.
Mine has gotten better with use, though it really hasn't had that much use. I bought the edger attachment for it and that stays on it constantly unless I use the pole pruner. The 4 mixer's have awesome torque down low, and if the line gets tangled in the grass/weeds etc, just pulling the trigger rips right on through it where a 2 stroke usually needs to be withdrawn first to get the R's back up.
If you want about the best trimmer you can get for trimming and edging, get the Husky 223L or 323L. The 323 is the best choice because it has a solid shaft and a little more HP. I have found very little weeds and grass that my 223 won't handle, and it is the lightest trimmer that you can find for commercial use.