I've been into scraping websites data using Python Scrapy although I have a strong experience in PHP cURL. I don't know which is better for scraping data and manipulating the returned values and the speed and the memory usage.
And what is (yield) function in Python Scrapy supposed to do?
Scrapy is a framework. You can define pipelines and systematic ways of crawling a URL; cURL is simply boiler plate code to query a page or download files over a protocol like HTTP. 
If you are building an extensive scraping system or project, Scrapy is probably a better bet. Otherwise for hacky or one time things, cURL is hard to beat (or if you are constrained to PHP).
PHP cURL vs Python Scrapy?