K...hmm let me see.
120 HP / 70 Atk / 105 Def / 95 SpA / 90 SpD / 60 Spe
Earthquake would help against Tentacruel, Tyranitar and most Fires (I'm looking at Heatran specifically)
Also, I'd like to emphasize how giving it Earthquake wouldn't hurt Tentacruel (since as you mentioned, the 180 BP of Woodhammer is only 10% less than the 200 BP of Earthquake) as much as it would hurt Fire types (especially Heatran) in general. Fire types are already considered to be strong checks against this, and eliminating that would be slightly sad =/
Anyway, to 2HKO 252 HP / 0 Def Tentacruel with Leftovers every time with a non-STAB Earthquake, you need 224 Attack.
252 HP / 192 Atk / 56 SpA Quiet nature yields 444 HP / 224 Atk / 246 Def / 261 SpA / 216 SpD / 140 Spe
(A pure physical set, btw, with WoodHammer would require 248 minimum Atk to 2HKO 252 HP / 0 Def Tentacruel with Leftovers...meaning of course you would have to run a +Atk nature and lower Woodman's overall effectiveness, a more acceptable sacrifice to beating Tentacruel)
With this set, I would run Earthquake, Flamethrower, Leech Seed, and Rest / Grass Knot. The last slot would be entirely dependent on my team...beat Gyarados everytime or not be crippled by Toxic?
This set 2HKOs Tentacruel everytime, and a STAB Flamethrower from a modest 261 SpA that directly complements Leech Seed and stops Blissey's / Snorlax's attempt to counter you. Even scarier is that with Earthquake and Howl (something it looks like this is definitely going to get), Woodman has a chance to OHKO 252 HP / 0 Def Tentacruel with 252 Atk EVs, whereas with Woodhammer that OHKO is never possible. It is various little issues like that cause me to hesitate extremely when viewing Earthquake as a viable option.
But even more worse than that is that Heatran's only hope to kill you is to first have one of its teammates die, and then run 306 Atk (252 Atk +Atk nature btw, lol) to hope to always OHKO this Woodman with Explosion. If it had Life Orb, it would still need 76 Atk EVs and a neutral nature to always OHKO this Woodman. What does this mean...by giving our Pokemon Earthquake...we have removed Heatran as a check completely.
Pretty much every other Fire, besides Arcanine (it has to run Intimidate), Charizard and Moltres, is 2HKOd by a 224 Atk Earthquake, assuming their standard EV spreads. If Stealth Rock is down? Forget about it.
My hesitation for Earthquake / Stone Edge is built predominantly on removing Fires as viable checks, but both also significantly hurt the rest of its checks.
With Leech Seed + Fire STAB, you can easily run a mixed set with less Special Attacking prowess simply because you are bulky enough to withstand a few hits, and Leech Seed provides some extra oomph. You can then pump the EVs into Attack and HP in order to simultaneously increase Attacking Prowess to 2HKO your threats and increase your overall defensive capabilities.
Giving this Earthquake and eliminating Fire Pokemon as checks would be a sad occurrence indeed, especially since one of the hopeful decentralization effects of releasing this Pokemon into the metagame was to increase the usage of Fire types in general. With Earthquake, this hope is destroyed.
