Asuras are fun overall. They do, however, have major problems with MP in PvE because they don't get MP regen from plate armors. If you have the pots though, they absolutely wreck. Ghost Orb is amazing especially if you can get it off with 5 runes. Asura Spirit Crescent and Fire Wave Sword also wrecks mobs.
PvP, they're known for being a one combo MONSTER. If you can go Upper Slash > Ice Wave Sword (Freeze) > Ghost Orb > Spirit Crescent > Fire Wave Sword, you can take off about 50-80% of the opponent's HP on that one combo alone. Wave Radiation also serves as a free GTFOff me whenever someone's too close. I haven't had much experience with Ground Quaker and Agni Pentacle.
Soul Benders are way different. They prolly should've traded off with Asuras somewhat with armors because they use magic attack but they're more suited for close range. They're easier on the MP due to having Cloth Mastery though. They're good party support in the fact that they can cast ghosts that give your party buffs and debuff mobs. Keiga raises your evasion and reduces the damage you take which makes using cloth more decent.
In PvP, I don't have much experience watching succesful Benders mainly because I tend to stick around Beginner a lot. Ashe Forke > Upper Slash > Full Moonlight Slash > Gore Cross > Ghost Slash seems to be a good combo though. I saw this Glad 1 Bender absolutely abuse that without even casting his ghosts. Other than that though, most Bender I believe cast ghosts to zone the opponent. Then they go Ghost Step for the invincibility then try to get in for a combo or Phantasmal Slayer. Tombstone whenever they can get the opponent OTG.
So yeah. Heavy bias on the Asura side because I quit my Bender early out of boredom / fragility <__<;
^ actually i have a friend that plays an asura (and sense he is lvl 45+) he did the Upper Slash > Ice Wave Sword > Ghost Orb > Spirit Crescent > Fire Wave Sword combo on me trust me on this if you got the right equipment that 50-80% becomes a 100% kill if u get that freeze and full charge your ball in time to use it to set up full charge crescent.
iv played both asura and soul benders and honestly both of them are boring at first but asuras get better quicker so id say go with it.
And one of the things that kept Asura exciting for me was the acquiring of moves. From 18-24, you have Neutral Wave Sword and Ice Wave Sword to play with. Not much but it's alright. Then 25 you get Ghost Orb and Spirit Crescent. This is when the Asura game really started picking up for me. 30 is the Spirit Crescent upgrade which is awesome because it makes your powerful SC even better because it can multi-hit. Then 35 is Fire Wave Sword which is an explosion of damage. Ever 5 levels for me was a gaining of a new awesome move so it easily motivated me to level.
I play both and after I hit 35 on my Asura I haven't stopped leveling him. It's at 42 now, and once I get out of where I'm living now where I lag all the time, I plan to hit 45 and switch back to Bender.
While benders get cloth armor, that's only 100% mana regen iirc. Unless you have +mp regen items you will still run into huge mana problems just like Asura. If you got the +mp regen items you could slap it onto your Asura and still be fine.