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What is the difference between the two?

There is no difference game wise, and the only reason I could see is that they are slightly different sizes (cover and all that).

Was there something different in the 3rd edition codex? (I have only used the 4th ed one)

I can understand using one or the other in your army for looks, if anyone ever used them.

It was just a redesign. The XV15s were the old 3rd ed metal models, and they decided to make them larger battlesuits and the metals were discontinued. Later (possibly because of DoW Dark Crusade) they brought back the XV15 since they were mildy popular.