I just noticed that Monarch & Fruity are made by one and the same company.
You are correct, we are the developers of both Monarch and Fruity. The reason that we're developing both has to do with the history of each product. We started developing Monarch about 18 months ago. It has always been a GroundWork property (though initially, it was a tool used internally to configure and support our clients' Nagios installations)
We hired the lead developer of Fruity about 4 months ago, and in doing so, we offered to support the ongoing development of Fruity. Right now, both are under active development (and Fruity is rapidly approaching a 1.0 release).
In the future, we may consider merging the two codebases into one unified product (with an upgrade path from both), but there are other priorities on our development schedule -- so for the moment, we are continuing to develop and support both products.
I'd encourage you to look at both of them, and determine for yourself which one fits your needs better. While they both solve more or less the same problem, they go about it in different ways, and we've found that people have a tendancy to prefer one over the other depending on the size/complexity of their network, and their existing familiarity with Nagios and its configuration.