用户 has the same meaning as 使用者 techlnically ,用户 is more formar than 使用者。

用户：“用”means “使用”or“正在使用”，“户” here means people；it's more simple and official; 使用者：“者”here means people.Because it use two word to emphasize the act "use", one more word than "用户",so it focus on the "using" . if you try to introduce a company or service,you might use "用户" to describe your company's customer group. if you try to introduce how to use a product in a illustration ,i think "使用者" is more suitable. However, you definitly can use these words interchangeably,even our Chinese we don't care to use it interchangeably.