My rule-of-thumb is Any Morale piece accounts for approximately +20-25 ilvl's (+1 ilvl per meld).

An i70 PvP piece roughly equals an i90-95 PvE piece. It really then comes down to stat-allocation & pentamelding beyond.

Never downgrade weapon-damage however.

As well, an additional consideration comes in playing a Healer or focus-target (BLM)... the additional tankiness. In their cases, I would say it's more like +30-35 ilvl's.