Just a thought, instead of them picking a song from a list of soul source suggestions. Maybe get them to draw up a list of their favorite soul songs they feel suitable for the occasion? I personally wouldn't want to pick a song that I've never heard before or a song that didn't have a personal connection with. Unless of course, one of these suggestions blows them away!
personally though, something like the Impressions - I'm The One Who Loves You or if they like 70's stuff, The Street People - I Wanna Spend My Whole Life With You