The XC7S50 is a Spartan7, which is supported by Vivado's WebPack edition (free but with many non-FOSS parts) as well as the FOSS SymbiFlow toolchain (I can't figure out which Place-and-Route tool that targets Xilinx 7-series parts they're using from the docs, but they imply one is at least vaguely usable) too.
The iCE40UP5K will work with any IceStorm-based FOSS toolchain (including SymbiFlow) as well as Lattice's free first party Radiant tool (and I think also the older IceCube2 toolchain).
It's a little odd that they're different vendor parts / on different 1st party toolchains, but it does look like you could program everything through the FOSS tools, albeit probably missing a few features (Last I looked a couple of the special purpose blocks in both the Xilinx 7-series and the UltraPlus were not fully understood by the FOSS stuff; that may have changed).
I've been meaning to try Radiant, I teach some classes on top fo Vivado + Artix7 FPGA dev boards, and it's so much less awful than ISE (Xilinx's old toolchain) I'm still pleased with, but there are some nice, affordable iCE40-UltraPlus boards coming on the market, and if Radiant is less of a hassle (Vivado is a 25GB install and not _entirely_ beginner-friendly) I'd consider retooling.
The Spartan 3/3E/6 lines are legacy parts (and hence stuck on ISE for programming, so you don't want them). The Spartan 7 parts were a late addition (not shipping until 2017) to the 7th gen line for the low end; they use the 7th gen blocks and tooling, but are "cost optimized" and lack the fancy transceivers and such - though irritatingly the cost and resources of the Artix and Spartan parts have a lot of overlap; Spartan7 parts run from about $15-140 in single, Artix7 parts from about $25-430.
