Is FDUSD vs USDT?

Could you please elaborate on the difference between FDUSD and USDT? I'm curious to understand the distinction between these two cryptocurrencies, particularly in terms of their functionality, use cases, and the platforms they operate on. Could you also discuss their relative popularity and market acceptance? Additionally, how do they compare in terms of transaction speed, fees, and security measures? I'm interested in gaining a deeper understanding of these two tokens and how they fit into the larger cryptocurrency ecosystem.