So im fairly new as well, still havnt done an intro post.. but my understanding to some of this is..
SBD is pegged to the USD so its value will always be as such. It can also be used on a site called "peerhub" maybe a few others as well. I feel like there is a bit more to SBD than that, Like its used to get SP?
SP looks like its great if you plan to be super active on steemit. The more you have = the more you will have returned from posts you contribute to. Which = more rewards for you and others involved in those posts. I beleive SP also gains interest which will counter inflation. Where as Steem and SBD dont (correct me if i am wrong there). So if your in steemit for the long game might be good to have most of your Steem currencies at SP.
As for buying Steem. I havnt done that through steemit yet. But i did buy some a few days ago when it was about 1 AUD on coinspot exchange and that worked out pretty good so far, its nearly doubled. If it gets that low again i will buy more.