mind plate or odd incense?
They both boost attacks by the same amount, 20%. However, the odd incense allows you to breed Mr. Mime and ger Mime Jr. and the mind plate changes Arceus' type to psycic when held.
Source: http://www.serebii.net/itemdex/mindplate.shtml and http://www.serebii.net/itemdex/oddincense.shtml
