IMO: there's very little difference in strength between having the "pulling" spokes on the outside as opposed to the inside. However, there's a good reason for putting them on the inside: if the chain comes off to the inside of the cassette (or freewheel), pedaling will tend to push the chain away from the hub because it's rubbing against "leading" spokes. If the spokes are "pulling" or "trailing", the chain will tend to be forced towards the hub, jamming it that much harder.

Wheelbuilding is fun. I must get back to the two sets I'm putting together now.