C++ or objective C, already knowing C

Hy guys,

I am quite familiar with with C. I am now looking for a language which would offer me a more sophiaticated toolset, such as classes and also a wider range of actively developed libraries etc... On the other hand I don't want to learn a completely new language currently. What are the main differences between those two (c++ and objective c) which one of them do you suggest learning/adapting. I've already searched the internet a bit but didn't find a comparison which also considers already knowing C.

I do most of the time mathematical based programming, analysis etc. but knowing a language which would give me a possibility to develope for Touchdevices and Apple-Devices in general would be very welcome.

What are your experiences and suggestions?