[nekochen]

If we are talking about Math-hammer...
Most of weapons have lower AP as the weapon strengh grows, so you need to take in the strengh vs toughness before calculating the saves.

Weapons fired by Marines:
HvyBolter 3 shots - 22% chance taking a wound w/o W.Field, 11% chance taking a wound w/ W.Field. 11% difference.
PlasmaGun 1 shot - 44% chance taking a wound w/o W.Field, 37% chance taking a wound w/ W.Field. 7% difference.
Missile Launcher 1 shot - 55% chance taking a wound w/o W.Field, 9% chance taking a wound w/ W.Field. 46% difference.
Lascannon 1 shot - 55% chance taking a wound w/o W.Field, 46% chance taking a wound w/ W.Field. 9% difference.

As you can see, the only REAL protection you are getting is from being pummel by Missile Launcher, and all the above calculation is based on a Tyrant standing in open field which I am sure no one would do such a thing... If I am to include cover save into the above calculations then even a 6+ cover save would make the W.Field no difference to one Tyrant without it... again, except the Missile Launcher.

Close Combat from Marines (regular troop):
Regular 1 attack - 2.7% chance taking a wound w/o W.Field, 1.3% chance taking a wound w/ W.Field. 1.4% difference.
Power Weapon 1 attack - 8.3% chance taking a wound w/o W.Field, 6.9% chance taking a wound w/ W.Field. 1.4% difference.
Power Fist 1 attack - 42% chance taking a wound w/o W.Field, 35% chance taking a wound w/ W.Field. 7% difference.

The only protection you are getting in close-combat is from a power fist of a 7% increased chance.

Basically the 35 extra points you are paying is only to give you the edge against a missile-heavy army. If you save 35 points here and more points somewhere else, you could even pay for a few more Raveners. 35 points is 1/4 of a Winged Tyrant's base cost, worth it or not.. it's really up to everyone's own perspective.