I've played both classes at 39 on private servers so ill try provide some insight. Both classes are decent at 39 and you can make them work.
enh shaman
As someone above said, their damage is based around RNG. Get a windfury proc and crit with PoD and you can pretty much 100-0 someone within seconds. I think the question you wanna ask yourself here is if you intend to play premades etc. or not. The high RNG burst is both a blessing and a curse; when it procs you're the king and when it doesn't you're the peasant. In premades/organized PvP enh shamans aren't super prefered since they rely heavily on luck/RNG and it's hard to land predictable kills, this especially matters for WSG obviously. With that being said, if you wanna twink and have a good time and blow people up enh shaman can be super fun (and good). You could also consider elemental shaman, which arguably is top 3 strongest specs in the 39 bracket.
rogue
Rogues can be played ambush/backstab or hemo at 39 – both specs are viable. At 39 you can have prep, imp sap, blind – in other words you pretty much have your entire CC toolkit available at 39 which is insane. Though this is compensated by the fact that rogue damage in 39 is kinda underwhelming imo. From comparing all the brackets (19s, 29s, 39s, 49s, 59s, 60s) I'd say 39 rogue is probably where your damage dealing feels lowest in comparison to other classes. Eviscerate is shit at 39 so you lack a deadly finisher. I even went full +AP gear on my rogue and I still don't feel content when comparing it to other classes' damage. I kinda feel like I'm painting a too dark picture here though: rogue's dmg in 39s is far from bad and shouldn't discourage people from rolling it, however you can't go around expecting to oneshot people and be a damage machine. I think 39 rogue must be played with CCs in mind because that's their strongest USP in this bracket.
I felt as a mage FC, I had a lot of movement with blink. Also, fire spec has blazing speed, I forgot about that
