I can’t speak for how good or bad an API Forge is, but you no longer need a modded client to connect to a Forge server. That was changed about a year ago, if I recall correctly. Also, when Sponge is closer to a release version, SpongeForge and SpongeVanilla will contain the same API features; essentially the only difference is that the Forge version can run alongside other Forge mods.
Lastly: I’m not 100 % on the legality of Spigot, either. They use a shady model of updating and distribution, and even so they kept all their servers in Romania the last time I checked. There’s something funky going on there.
EDIT: Talking about Pore, it will most likely not support plugins that heavily utilize NMS. Other than that, most functions will probably have a Sponge equivalent (even if the input has to be changed before it’s fed into Sponge), so a lot of plugins will run. And then there’s the fact that most major (and a lot of less used) plugins are being ported or rewritten for Sponge anyway, so you’ll be able to run those instead. There’s no doubt that Sponge is a much more powerful API than Bukkit (and thus, by extension, also Spigot) was.