I will start by talking about each of the two platforms and compare them together tell you the advantages and disadvantages to be a ble to decide which one is better for you to work on
Let's start with fiverr
In fiverr everyone is lowering their price as low as possible in order to get clients
From its name fiverr is coming from five so basically most of the time you will be underated
But that can change, if you have already got some work done on the platform and good rating you will start getting badges and you will be able to get more clients and consequently increase your price rates
But that takes alot of time and effort
On the other hand upwork is really professional it doesn't accept immediately anyone to just register you must comply with their rules, they don't want anyone to just wakeup and say I am a professional and register, it takes a process. Most of the time you are going to get accepted immediately if your work is professional.
But upwork's disadvantage is that you need to buy what is called connects. These are like coins of the platform. In order to give someone a proposal that you can do their work you must use some connects.
In conclusion, fiverr is easy and not really professional and most of the time has low rates and upwork is a little bit harder and more professional but it needs you to be a professional.