When you land at Félix Eboué Airport near Cayenne, the obvious move seems to be a local SIM. Orange runs the strongest network in French Guiana, so a local card is not a bad idea on coverage. The problem is everything around it.
An Orange SIM from a shop in Cayenne usually costs around 15 euros, and that is before you load any data. You often need to show your passport, the staff may speak only French, and the boutique might be closed when your flight gets in late. You also have to pop out your home SIM and keep that tiny card safe for the trip.
The MegaEsim plan runs on the same Orange network in French Guiana. You get the strong signal without the shop visit, the paperwork, or the language barrier. A 3 GB plan for 15 days is $2.10, which is far below what you pay for a local starter pack.