More than anything, it depends on your requirements. While Size, Weight and Power (SWaP) are the main drivers for ICs in-general, if you aren't compelled to develop an ASIC because of those requirements, Performance is your next consideration, which may push you back to an ASIC anyway, but, you may be able to use an FPGA if you can afford the SWaP trade-offs.
- FLASH-based FPGAs require "no time" to configure, as they are "instant on". Your design may require this.
- FLASH technology is lower power than SRAM
- FLASH-based FPGA doesn't need a BOOT PROM, thus one chip vs. two (or more).
- You may have a requirement to power-up in the previous state.
- FLASH-based offers more Rad-Tolerant solutions. There are ways to deal with radiation requirements, or SEUs in-general, in SRAM-based FPGAs, but, Microsemi offers "hardened technology"
FLASH-based FPGAs (Actel, now Microsemi), traditionally, have not had the density or performance one could achieve with SRAM-based FPGAs, so, if performance was the driving factor, you would choose Xilinx or Altera (now Intel), or maybe Lattice.
Essentially, you are driven by the requirements of your system, and your IC specifically. Early-on you address these requirements and perform a trade study of the different FPGAs (spread sheet). SWaP and performance, followed by recurring cost are the main considerations you want to iterate on with your team (systems, CCA, maybe even SW) that is fedback to your project Chief engineer/manager. Other concerns such as reliability, manufacturability, etc. are usually provided by other team members from those respective organizations, but usually don't mean much without your initial trade, and typically won't prevent your choice.
There are articles on the web if you search "SRAM vs FLASH FPGAs", but you will likely learn more from a trade-study against your requirements using the data sheets than you will anything else.