The first is more precise. If someone tells me they will be here in 20 minutes, I expect them no later than 20 minutes. They would be late after 20 minutes. The second is an undefined amount of time. That is, they may come at 25 minutes, 30 minutes, etc. Essentially , they’re saying it will take them at least 20 to get there, but they don’t know how much longer. If you want to give a range, it would be: I will be there between 20-30 minutes. BTW, another way to say #2 is: it will take me at least 20 minutes to get there.

First word - you are set the some place to go destination show the time, you will be there in 20 minutes . Second word - you have a meeting at 12pm then you are say to your group members it's 11:40am, i will be there after 20 minutes.

To me they mean the same thing! But I'd think you are more fluent in English if you use #1.

How to write My delivery boy reach in there 20 min