SEMrush is a “tool suite” that has the ability to do KW research. Keysearch (I assume this is what you really meant) is a tool devoting EXCUSIVELY to just keyword research. SEM Rush doesn’t have a Google API, it uses “clickstream data” to populate it’s data. Keysearch does have an API, so it’s data tends to be a little more accurate. It’s not a “guess” based on 3rd party metrics. Keysearch is 1/3rd the cost, easier-to-use, and extremely effective for bloggers big and small. If your goal is to work smarter not harder, Keysearch is the better option in most cases.