The R-Pi is a lot more flexible than the Arduino, however the R-Pi is a bit lacking compared to the Arduino. But there are plenty of options for the R-Pi like the Gertboard as an example.
I was one of the original right at the front of the queue people for the R-Pi and did a fair bit of early firmware testing and wifi troubleshooting (and some Citrix stuff for work) but as soon as the Rev2 came out I had one of those too it’s more refined and the Raspbian distro has come on leaps n bounds and is quite usable now.