At lower levels they could both solo pretty fast, the more twinked the better. For level 60+ though I think you would be able to solo better with the beastlord.
SK good - Good armor, half-decent combat spells, FD, snare, tap heals.
SK bad - low dps, medium downtime refreshing hp/mana between fights, no slows, weak buffs.
BL good - good dps with pet, half-decent combat spells, slows, heals, decent buffs.
BL bad - weak armor, no snare without pet aa, no crowd control (mez / root), no way to split (pacify / FD) now that pet pulling is nerfed.
The biggest drawback I see using beastlord is just trying to find the right camp to solo. You would want mobs that don't run unless you are pulling to a nice big open area with no adds around or if you have the pet snare aa. You would want to find camps where mobs are single pullable as much as possible or else break a camp so you can pick off respawns as they pop. With enough AA beastlords really crank out the dps.
The biggest drawback using shadowknight is just that you don't have slow available. At level 60-75 soloing will get harder and downtime will be longer as you fight harder hitting mobs. You'd be mostly kiting to solo good xp mobs or mass killing low xp mobs.
If you're used to soloing with a necro the kite method for SK should be easy enough for you. Beastlord soloing is all fighting though, you and your pet. BL pet lets you pay less attention while fighting easy mobs, tough mobs you want to be on your toes of course.