The Fs 100RX is generally the one everybody picks because of its lightweight its only 10.8 lbs. It has the same engine as the 110R both are 31.4cc the 90R is 28.4cc the 110R weighs 12.1lbs:thumbsup: the 90 is 11.7lbs. The Echo 225 is 12.1lb.
Like I said the big difference is the 90 and 110 both have a solid drive shaft and the 100 is a flex cable I believe. Personally the weight difference between the units doesn't make a difference to me.