Arduino and Raspberry Pi are cheap boards that receive and feed data to other computers, enabling the performance of various functions, both simple and advanced. Both are used to write and upload instructions in the form of code.
Some just cannot choose whether to get an Arduino or a Raspberry Pi, thinking that this is like a debate whether to get an Ubuntu or a Fedora, or a Mac or a PC. These are essentially different with each other, as the Raspberry Pi is a computer while the Arduino is a microcontroller, and may end up being useful together.
Or maybe the question is what to give a beginner first. To answer the question, it all depends on the purpose. Whether one is a beginner or a parent wanting to teach basic electronics to a child, they may choose to start with an Arduino or Rasberry Pi. Answering these questions can narrow down the options in deciding what to buy:
Does the project contain parts necessary to complete it and are these easier to acquire than those of the other?
Arduino shields or specialised add-ons are vastly available and these could be anything you can imagine. The Raspberry Pi sources is growing, but some parts can be uncommon.
Will one need the built-in HDMI, Wi-Fi and USB ports of a Raspberry Pi?
These are computers that need to be connected either through Ethernet or Wi-Fi. Raspberry Pi provides a no-brainer built-in ports. The Arduino, on the other hand, is capable of acquiring all these functions with shields, and they are available in shops such as Geek Studio, which caters to many other boards and brands.
What is the goal? Is the project aimed at learning programming or repurpose the device?
For this, Arduino can only be good for one thing and performs with less efficiency in multiple functions. Deciding to move on to a more complicated project may mean buying another board. The Raspberry Pi can be used as a general purpose platform for numerous applications and can even contain hardware hacking.
As it is not a debate whether to get a hammer or a saw as a tool, so is deciding between an Arduino or a Raspberry Pi. Like mentioned above, some may find these useful in one project with or without the other.