I love this question - it's like asking "what should I learn first, snowboarding or skiing"?
I think it depends if you want to snowboard or to ski. If you want to do both, you have to learn both.
In both sports, you slide down a hill on snow using devices that are sufficiently similar to provoke this question. However, they are also sufficiently different so that learning one does not help you much with the other. Same thing with C and C++. While they appear to be languages sufficiently similar in syntax, the mind set that you need for writing OO code vs procedural code is sufficiently different so that you pretty much have to start from the beginning, whatever language you learn second.
