They are both highly useful and wonderful in their own ways! They both greatly help parties, crec through debuffs and SM through buffs, though you are correct in that crec are damage based and SM have heals. They can also both AoE quite nicely, and soloing through leveling isn't hard.
I'm more familiar with SM (I have one at level 50) - I personally wanted it to be support, so I left DPS skills at 1 (other than taking up max awakening and immunity, since it does a heal burst as well) and maxed links/leveled some of the heals. Many people recommend leveling as DPS then switching builds at 50 with the free skill reset skill, but since I'd rather hang onto mine just in case I need to change build later, I leveled with weak DPS. In raids so far, my DPS has been horribly low (I spam some dps skills to try to crit to buff my links, though I heal as needed as well), below even healing threat.. but I'm also way lesser geared than the others in the raids I've gone to. However, even SMs I've seen with leveled DPS skills tend to be at the lower rung of threat, unless they're in form (then they sorta keep up to DPS classes).
As for crec, I've just partied with one regularly - I actually did dungeons as a SM with my friend's crec.. duoing is super fun. <3 They have different types of builds, too, so hopefully someone will weigh in on the differences. There are several DoTs they can pick up, but I think the really shining point of them is their two options of debuffs - either make the target take 15% more damage, or do 15% less damage. Being able to have someone switch that up as needed is quite helpful, and has allowed weaker parties take down raids with ease. Also, when transformed, they can buff the party's defense. I have seen my friend do some pretty impressive tanking in a pinch, too~ Threat-wise, he at least keeps up with dedicated DPS (I feel like he matches threat really well with my rogue, when we were similarly geared), though I don't know how this holds up in general with the class.
As for the guides, they're fairly nice? I know I read through all of the topics relating to SM when deciding how to build mine, and they were pretty helpful. I haven't raided enough on mine yet to really get a feel for how I like my build. There are a couple crec topics out there, though you could just ask on what's unclear and I'm sure someone will help you out.