Which is better??? Oooo I wish I knew!
There's a lot of factors that come into play here. First of all, Venom gets a boost in Hell difficulty. On West Ladder (where my Ghost was originally made), all duels were done in Hell difficulty. TienJe's ghost was made on West NL, where they typically have priv duels in Nightmare. Venom does slightly less damage in nightmare, making physical more desirable for duels there (in my opinion). Also, physical ghosts make better teammates with necros, since they benefit more from amp.
Whats the difference between a physical ghost and a venom based one? Usually they have similar damage on Fury side, on Chaos side the Physical ghost will display around 700 more damage than the Venom. The similar fury damages are deceptive, a larger % of the physical ghost's damage is of course physical, which takes only a 50% penalty vs. an opponent as opposed to 75% (assuming the opponent has maxed DR and PSN RES). On the otherside of the equation, the Venom based ghost's Venom benefits more from the 25% double application bug. Also in Hell duels the penalty to opponents PLR will mean that Venom based ghosts will do even more damage.
Both Ghosts benefit from Deadly Strike, the physical ghost benefits more. Physical ghosts usually stack more deadly strike, giving them even higher damage.
Venom Ghosts have higher shadow skills in general, their mind blast does more damage, they have an easier time having max DR, they get a higher claw block % (mine has 61).
What it really comes down to is preference. Do you duel in NM or Hell? Do you like a slightly more caster-like Ghost (venom) or a more melee oriented one (physical? Are your triwhirls perfect (physical benefits more from a perfect triwhirl due to the frequency of hits)? Do you often team duel with a necro partner?
At the end of the day, I think that it comes down to preference. It's worth noting that physical Ghosts require a high damage eth bug claw, which is rare and expensive. My Venom based ghost is doing just fine dueling the same people as TienJe's ghost (he does better, but that's down to him being a better player as opposed to his build) in nightmare, so your choice is really between two very similar builds in terms of effectiveness.