I am pretty sure there is a spell-check function built into the "post reply" submission box. I think it might be dependent on language sets installed on the forum (we are running the default phpBB British English language pack). My errors are flagged on the fly with recommended spellings and auto-change when the word is complete if it is known to be wrong. I'll have to dig into further settings if you would like more specific info.
As far as add-ons, so far I think it is a bit slim. The old 2.0/3.0 software packs had tons of mods and styles available (check them out here: https://www.phpbb.com/customise/
and here: https://www.phpbb.com/customise/db/modifications-1
). The new 3.1 software apparently doesn't play so well with the old stuff so I think it will be awhile before we see developers revising their code to work with the new software. I'm sure the more popular mods will be updated to extensions first.
I am only slightly familiar with Simple Machines Forums, and mostly as a user. I am a member of only a small handful of sites that use that package. These days it seems like the largest forums or those with corporate owners/advertisers prefer to run vBulletin. When I moved this board over from the ancient ezboard site, I considered using SMF due to good reviews. However, the process of getting data from ezboard to SMF was more than complex. Tim and I also discussed vBulletin due to their support services, but the cost was prohibitive.