Fs55 and fs94 are both 2-strokes. 55 series is homeowner and 94 is a pro model. Fs90 has a good deal more torque than the 94 but "sounds funny" as it's a 4-mix. The fx100 is basically same as fs110 with a hollow drive shaft that isn't rated to drive attachments like the polesaw etc. The 90 will handle most attachments, but the fs110 and 130 are each a step up in power. The fs110 is the same engine used in the ht101 pole saw and the 130 is the same as the ht131 pole saw. Weight and price are so close many opt for the fs130.
The good, the 4-mix engines all start very easy. Usually one or two pulls and off they go chugging along like mini diesels. Your wife may appreciate the slight weight advantage the fx100 gives with the Hollow drive shaft if it's only duty will be trimming. The other side effect of the Hollow shaft is slightly better throttle response due to less rotating mass. Personally I didn't find it very noticeable if say the fs90 is properly tuned. Slightly better maybe. Only the power mongers among us will want more than the fs90 offers. It really is very strong. The fs130 is an absolute beast of a trimmer.
The bad, really, there isn't much bad about 4-mix engines anymore. Early models had some hiccups, those have all been addressed that I know of. Solid reliable machines now. I'm still no fan of the Stihl trimmer heads. Many opt to replace it immediately with a Speedfeed head from Echo or there are several variations like an Oregon.
