As Heredurian said barb is good against bosses without mobs, with mobs it's problematic. However a good barb can do any dungeon in the game, but for a dk Techno is easier. Bossing wise, such as elm, is also easier for a dk and ofc warden.
For what concerns experts, stone def as first is a safe bet. As second it depends: warcry helps tanking, usefull if you want to start to tank tower and lab. On the other hand combat fury helps as it restores hp. However if you intend to power quest till level 28 i suggest taking a damage skill, defeat or shield bash. Ideally at 28 you would want: stone def, combat fury, shield bash and defeat regardless of pvp and pve. Another must is warcry for pve. Immunity is a must for pvp. Last wish is good in both and by being passive, it's good to have, however not a must. New skill is optional, some barb likes it, some not. It's a nice boost to block, however i tried it and don't like it.
As a pve barb my skills are 5/5 strong blow, 3/5 roar and 5/5 taunt, 4/4 warcry and 4/4 stonedef. A bit overkill on the taunting department, but since i'm not full +10 and have full block/parry gear, i need it to prevent aggro from top dps. I'd love to remove some points on the taunts and improve my dmg, but for this i'll wait for more amp. The reason for strong blow 5/5 and not roar 5/5 is personal. I like the dmg, it helps with questing, bossing and hp regen (i have 7.5% health restored on dmg of strong blow).
If you plan on soloing bosses only i would go 5/5 strong blow, 5/5 roar, 3/5 chop, 4/4 stone def, 4/4 defeat or shield bash or combat fury (if you struggle). However this build is trash for tanking dg imo, but good soloing. If you are amped and have good gear, you wont need a specific soloing build.
Many barbs claim that they are able to solo bosses and tank dg in pvp spec. This is true. However they know what they are doing, maybe use consumables/minions and have optimal equipment and amp.
Despite the shortcomings said of the barb compared to the dk in pve, he is a beast in PvP.
Therefore if you plan on doing PvE only, you can still choose barb ofc, but dk is optimal. Since dk can do all what a barb can do and more. And i say this as a barb.
It also depends what kind of content you want to solo. Barb is good for 90k bosses and down 80k bosses. However even a dk can do this. A well amped dk can also solo new lab bosses. Not sure a barb can do this, will test once i'm stronger.
