If your looking for light weight, the Echo CS 361P is a nice little saw for the money. For the small stuff. (say up to 12")
Well, the way I figure it is there is an ergonomic convenience, or ability to a top handle saw, that a rear handle saw just can't do. Just as a rear handle is better suited to cutting where leverage plays a factor. So if you already have a pretty good set up of rear handle saws, it would make sense for you to purchase the top handle saw that makes the most sense to you.
Nothing beats a top handle when you are in a tree or topping a tree thats down where you want to hold on with another hand. That one hand off to the side as you hand on it the other. Almost possible with a rear handle. With a top handle I dont go around one handing it though often but even you need to you have the control.