As long as you continue playing high vitality version of this build with fortress, wolf companion is vastly supererior to spirit guide. Curse is added to every hit and every proc that deals physical component, at full value, its not divided by %wdm. So you get it added to every 60% wdm deathgaze axe, every 50% arrow, always 1500+ damage to all of these hits. If you happen to have more procs, wolf set pieces, bladestorm, crucify from gloves, all these have even lower wdm, yet, wolves add full value to each of those hits.
Another argument against spirit guide is, you only get 10%ed per soft point, thats very poor scaling. Defense shouldn't be needed, if you do everything right you gonna have 1mil+ defense, and at lower levels, you have less, but for those ubers even that 700-800k should result in 5% chance to get hit.
EWD bonus loses value the more strength you get and more buff bonus you have (bloodlust from merc), because you will add on top of already big bonus.
There is a a point where I recommend ditching both wolf companion and fortress, but thats in the lategame, when going full strength, but I recommend putting those points into daedalus, you gonna get 1-3.5k lightning damage and about 2000 flat life, which results in 20k hp with 0 vitality invested. When you are carrying all charms and really good items, with full str setup, your d2stats phys damage exceeds 20k, then having +1800 wolf curse is less significant (it is still better than bonus from daedalus or spirit guide would be), but you just trade that part for passive that also gives you health.