We have done it both ways. US/WDW in Aug '01 and WDW/US in Aug 03. WDW first is my choice. We find we are usually a little drained towards the end of the trip after going to parks every day for a week. US/IOA is much less stressful especially when staying on-site with FOTL access. The parks are smaller and you do a lot less walking and standing in line. The slower, easier pace at US, which includes the ability to walk to the parks from your hotel, was a nice end to our trip this year. If we do a split trip again, we will do WDW/US.