UNL has a few new storylines, new music, new characters, and a "comeback" character (China), but other than that, it is a patch for SWR, so yeah o3o
Unless you really want to play Sanae, Suwako, Okuu, Cirno, or China (or hear the music; I honestly like the remix of "Faith is for the Transient People"), then you don't really have to have it.
Although a lot of people say IaMP is the best. It looks really good, but Idk. I've never been able to get my hands on it.
/ohohoh, and this is irrelevant, but at the anime convention I attended last year, I cosplayed as Marisa (though the outfit sucked, but that was because it was a last minute put-together), and I bought a copy of SA there