When to use 相比 and 比?
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.

For example:


  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.