I've done some pretty harsh edging with the string attachment on my 90, and it wasn't lacking at all. With the stick edger attachment, it has more than enough for anything. I did an edging job with the edger attachment this past spring where I had to cut the grass and dirt back over a foot on average, and a foot and a half in some spots, and cut down 3-4" to reach the hard surface below. I expected it to bog some on that, but it cut everything with ease. I was really impressed.
If you can afford the much more expensive 130, there's no harm in going that route, but the 90 is all you really need for anything in the business outside of doing a ton of work where you have to cut down 3" tall thick grass and weeds. Even then, the blade attachment will cut it down with ease with the 90.
According to Stihl's site the FS90R weighs 11.7 lbs, and the FS130R weighs 12.3 lbs. That's over a half pound more, and you may, or may not feel it at the end of the day.