After using raspberry pi and arduino, and with some suggestions from dc member Edvard, i finally have a better understanding myself of Raspberry Pi vs Arduino use -- at least for the kinds of projects I'm interested in.
At least in my case, the Raspberry Pi is just a small but fully capable linux pc with some input-output abilities. Good for when you want to be able to do full
normal development coding for a small standalone
The Arduino, for MY purposes, is much more suited as a little helper
device that can be coded with small programs and is well suited for working in tandem communication
with a full pc.
So for example, in my dicer
project, the Raspberry Pi was a reasonable (although slow running) fully standalone computer that could do everything for the project -- run the software and drive the hardware.
But the Arduino turned out to be more useful to me, at least during development, as a helper device so that my Desktop pc could run the software at full speed, and communicate with the Arduino to do the hardware control.