In which niche do you want to develop yourself like a freelancer?
Maybe there is some freelancing area you will not find in one of them. You could have both of them, and nothing can stop you from using more platforms if you want to work like a freelancer.
Both. Create Fiverr gigs - do not copy and rewrite someone's gig either, I've noticed a lot of that laziness going on recently.
Sign up for Upwork, verify your account, and get your profile 100% or as close as possible. Create "Projects" - which are basically Upwork's version of Fiverr Gigs. Almost identical form for creating the gig/project.
Seriously do both and apply for as many jobs on Upwork as possible. You can always add more connects to apply for more jobs as you earn. You will earn if you put in the work to apply for jobs properly.