FireWire is regarding a lot of manufactures history. Only a few make accessory stuff (e.g. Universal Audio) for FireWire 800.
Apple, the initiator of FireWire declared it obsolete many years ago now.
That said, it a great and very stable interface. I have always put a lot of trust in my FW connections and have avoided USB for audio connections. But I have the impression USB has become better over the years. I now even have a mobile stereo USB box in handy for quick location stuff (the AudioBox iTwo made by this company). It is OK, do not know about latency, well.. it is only 2 in, 2 out anyway.
Track counts will tell what you need, the SL 16R only can get 18x18 over USB. OK, the 32R offers 40x40. Over FW800 you can get much much more, and the way the interface works is that it always reserves the full bandwidth. A tiny downside is that your computer is very busy on a continuous base, leaving it pretty warm. I noticed that with the Thunderbolt to FW800 interface on newer computer this is not the case any more.
But.. the 16R does offer AVB. Maybe that is something to consider. If it is already implemented. I have the older ecosystem and for that AVB never came. So please check the current operability of AVB first! I cannot tell this, not owning any new SL III stuff.
One of the advantages of AVB over a great track count and good latency values is the distance of the connection, which USB or FW could just put the interface a few meters away from the DAW, with AVB over a Cat 5e or Cat 6 connection easily goes 50m (and even beyond to my experience).
If you are not going to use it as a mixer and only as a recording interface, then you could also consider the new Quantum interfaces plus expansions. They have the very-tight-latency Thunderbolt II interface. That is if you go the record-only route.
Small hint on the side: Always get Cat 6a (mind the a!!!) over Cat 6 for its downgrade compatibility with Cat 5e connectors.
