I've heard it said that Shapechange is better than wild shape. Is that true in epic levels?
Shapechange: It caps at level 25, but it does what you'd need a bunch of wildshape feats to accomplish. If you can argue your DM into allowing Enhance Spell to work on Shapechange (to increase HD), then Shapechange is probably better forever (combined with Improved Metamagic and super-high-level spells).
Epic Shapechange: Is there any way to create such an epic spell, using the Transform seed?
Wild Shape: No limit on hit dice, but it gets harder and harder to find an elemental, plant or animal that has enough HD. Wilding Claps let you use items which aren't accessible in most shapechange forms. The epic feats Dragon/Magical Beast Wild Shape, Gargantuan/Colossal Wild Shape help.