I'm not too optimistic about the Armada's prospects. Especially compared to Sporting Club Jacksonville. The MLS Next League they are joining is basically just the reserve teams for MLS clubs. There are only two clubs currently in the league that aren't feeder teams for existing MLS clubs. The primary function of the league is to provide competition for youth players as a test to see which players (if any) are ready to get called up to the MLS feeder teams. But by and large most players in the future in MLS will not even come up through the league. Like in Europe, maybe a few players on the senior teams will have come up through the reserve team.
MLS Next is the third division, while SC Jax will play in the second division in the USL. With the direction soccer is headed in the country, the second division (like in England, Germany, and other European countries) can become fairly competitive and popular. The average attendance in the USL in 2023 was 5,552. This is more than the Jumbo Shrimp drew last year and over 1k more than Minor League baseball averaged. MLS Next games are mostly scrimmages with like 100 people at games.
Lastly, SC Jax is affiliated with Florida Elite, which seems to be the far and away dominant club in Jax now. Armada is affiliated with JFC. When I grew up playing there were basically four competitive clubs in Jax: Clay County, JYS, JFC (which previously had different names), and Jaguars (which predated the NFL team and got a nice settlement). Jaguars are now defunct (RIP), JYS doesn't have travel teams, Clay County and JFC are both weaker than in the past. Florida Elite is a newer club, but is already totally dominating the soccer scene in Jax.
Basically, I would bet a lot of money that SC Jax will be the more successful of the two clubs.