Hi @ronald - the direct answer to your question is you can use %user_input% and %Variable%user_input% interchangeably. In other words, they both produce the exact same result: the value of the variable in question. (IMPORTANT: you must capitalise the "V" in "Variable".)
The alternative answer is this: you should never use %user_input% to get the value of your variable. You should always use the form %Variable%user_input% instead.
Why is that?
Well, as you know KM provides many in-built text Tokens such as %SafariURL% and %TriggerValue% and %UserName% - see the whole list of them in the KM wiki
Because of this, it would be easy to confuse your use of the variable token %user_input% with a KM text token - simply because of the way they look similar. Several KM experts in this forum have said that it is always best to keep potential confusion at a minimum and make sure that when you want to get the value of one of your variables you use the full form which is %Variable%user_input%
It is up to you which form you use, though. Personally, I always use the longer form, %Variable%user_input% even though it means more typing!
I hope that clarifies things for you a bit.