Consider first melee auto-attacks. Your weapon dps is increased by roughly 7.14% of your Attack Power. That means that if your weapon does 295 dps and you have 1700 Strength, your actual dps from auto-swings will cause ~563 dps reduced by your total chance to not hit (miss + dodge + parry). So, if you are soft-capped on Hit Rating, but below the soft cap on Expertise, say at 22 Expertise (1% dodge + 9% parry = 10% chance to not hit), your auto-swings will do 506 dps.
Now let's try adding Str or Exp in appropriate measure. One full epic red gem will give you either 20 Strength or 20 Expertise rating. 20 Strength will give you 44 additional AP, which will increase your dps by 2.84 (= 44.1 AP * 7.14% * (100 - 10% chance to not hit)). 20 Expertise rating will become 0.65 Expertise skill, which will reduce your chance to miss by 0.325% (0.65 skill * 0.50% dodge + parry chance), and increase your dps by 2.30. So on auto-attacks, the raw Strength offers more value.
Alternately, consider your Heart Strike ability. HS hits for a percent of instant weapon damage, meaning weapon speed is now a factor. From the base values above and a standard 3.5 sec base swing speed, your instant weapon damage is increased by roughly 25% of your AP. HS uses 100% weapon damage plus 735, and is increased by 10% for each disease on the target. If we assume both diseases are up, that means that the HS will do 3246 dmg on a successful hit to its main target. If we average that value over all your attacks to account for the strikes that do not land, that is an average value per swing of 2921. Now add a pure epic Strength gem again, and your HS damage average per use will increase to 2933, for a 0.41% increase. If instead you added a pure Expertise gem, it would reduce your chance to not connect and give an average per use value of 2932, for a 0.38% increase. Strength still offers very slightly more overall value.
A key ability to consider now, however, is Rune Strike. Rune Strike cannot be dodged or parried, and double dips from attack power by using both a percentage of weapon damage *and* an amount of AP as an additional value. These combined mean that your RS will receive no value from Expertise, but will receive a lot of value from Strength. Rune Strike will constitute a significant portion of your threat.
Spells will also not gain value from Expertise as they cannot be dodged or parried, but they will receive a small portion of your attack power in their damage value.
Combine the above and the result is that Strength will offer more total value towards your threat than Expertise in total damage, but Expertise will give a stronger likelihood that when you press the button, you get the value of the move. The margin is narrow enough that it would not be a significant loss to add Expertise rather than Strength, and the reliability factor gained may be more desirable.