Any weapon you customise won't have the benefits of ignoring the target's Defence when attacking physically, nor will they be more powerful when you have more/less HP/MP.

The only case where a customised weapon is better than a Celestial Weapon for dealing damage is when casting magic. Since I can't be bothered to get Lulu's Sigil and don't use her for physical attacks, I always customise a specialised magic weapon with BLD, Magic Booster, One MP Cost and Magic +20%.