A common question that gets asked about this project is "What are the advantages of using Cello over C++ or Objective C?". The short answer is personal preference. The features of each are fairly similar, but they all do differ in other areas.
The first Major difference between Cello, C++, and Objective C is that Cello is just a C library. The others are whole new languages.
Many people prefer programming in C to C++ or ObjC and many people prefer the opposite. This preference is something that may influence your decision before thinking about anything else. You may only have access to one compiler type or are required to use one language.
There are two major difference between Cello and C++. The first is that Cello must rely on void pointers and runtime polymorphism to achieve its high level features, and the second is in the design and structure of the standard library of each.
The standard C++ library is well known for being large, complex and spanning paradigms. It supports many many things out of the box using different methods and techniques. Much of it is Object Oriented (such as IO), while other parts are Template based (such as STL). The C++ standard library is like a huge messy toolbox.
The Cello standard library has been designed from scratch. It is much smaller and modern, with consistent ideas and design. It is designed using Composition rather than Inheritance.
Cello also comes with a basic form of Garbage Collection.
I don't have nearly as much experience in ObjC but I can tell you that it is fairly similar to Cello when it comes to internals. The major difference is in the syntax and type checking ability.
Objective C, like Cello, has a runtime system built over the top of the normal workings. It also does dynamic function calling via a message passing type interface. The ObjC type system is both runtime and compile time giving it the advantage of being able to do compile time type checking to ensure objects recieve valid messages. This adds valuable type checking but can sometimes make some task laborious. I find Objective C often requires lots of boilerplate and that the Apple libraries are full of Object Oriented style design patterns. Objective C's syntax is generally regarded ugly and complicated but overall it is an interesting and powerful language which I like a lot.
If you prefer OO style design, with patterns and boxes, and like to have types checked at compile time, then Objective C definitely offers you more. But if you primarily like the dynamic nature of Objective C but wished it was more flexible then Cello may appeal.