When comparing Iron vs Nickle, the Slant community recommends Nickle for most people. In the question“What are the best web frameworks for Rust?” Nickle is ranked 6th while Iron is ranked 11th. The most important reason people chose Nickle is:
By default nickel catches all errors with its default ErrorHandler and tries to take reasonable actions. In cases where one wants to provide an own ErrorHandler (e.g. for custom 404 pages), it's trivial to write one
Ranked in these QuestionsQuestion Ranking
Pros
Pro Not longer actively maintained
See here.
Pro Custom error handler
By default nickel catches all errors with its default ErrorHandler and tries to take reasonable actions. In cases where one wants to provide an own ErrorHandler (e.g. for custom 404 pages), it's trivial to write one
Pro Easy Templating
Nickel supports defining templates with mustache.rs. All you need is to create the template
Pro JSON handling
nickel makes it easy to map JSON data right onto your struct
Pro Middleware
Middleware are the extensibility points of nickel. Batteries included! A bunch of existing Middleware comes right with nickel
Pro Flexible routing
Routes can be as simple as /foo, use parameters, wildcards or even double wildcards
Cons
Con No longer actively maintained
See here.