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well, that would depend on your criteria (what specs you need) what is the project? mostly, you need to be concerned about how many i/o pins you have to work with. Also, I have to point out that there are tons of arduinos out there to choose from, so that is very general. The second thing is processor speed, which probably won't be an issue, unless you're doing super high speed computing. i just did a project with an Uno (arduino) that took 13 digital out pins. the uno was fine for that. the raspberry pi is much more powerful and is more like a computer. you can even hook it up to a display and use it as a computer. so short, if an arduino will work for you, it is the smaller, cheaper alternative, while the raspberry pi is more for making your own microcomputer. sorry that I'm answering a question that is off topic, but I wanted to answer it anyway. tl;dr You need to be way more specific in your question for me to be able to help sufficiently.
OK thanks @J.G. And @Rainer Wess.. I only need to control things.. And my project is just the basic one no high speed computing required 😅so I will go with Adruino....
If you need a powerful cpu for audio,video or games, than raspberry pi is it. If you want to control something, a little machine a robot, or do home automation, read sensor data, then arduino should be your choice. In short Raspberry for computing power, arduino to control things because its has more gpio. (gpio = digital and/or analog in and output channels)
use both. Arduino clones nowadays range in the 4$ price tag