I'm not sure to get the difference. If the two following sentences are correct, when should I use one over the other?
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It's quite easy to distinguish.
First is the conclusion:
"相比" in Chinese equals to "compare with" in English.It's usually a common verb.
"比" in Chinese equals to "more than" or "xxxer than" in English.It's usually a comparative form.
Their usages are quite different.
you can use "比" like "我比你高。(I'm taller than you.)" and "相比" like "农村的文化生活无法与大城市相比。(The cultural life in the country cannot compare with that of a large city.)".
Hope my answer helps you.