There are a couple of questions you have to ask yourself.
How long do you intend to be at Epcot? If you're planning on being there the entire day, then you should know that FW generally opens and closes about 2 hours earlier than WS.
What are "must see" attractions? If you absolutely have to ride Test Track and Mission:Space, then you will want to see them early in the morning or later in the day (or use FastPass) - this will have an impact on how you "do" Epcot.
Are you interested in the street shows? There are a number of shows that take place throughout Epcot during the day and many of them are in WS. The times guide will indicate when and where. We really like these and will organize part of our touring plan aound this.
Do you have a dining preference? There are a lot of great restaurants in Epcot, particularly in the WS. If you want to eat in the more popular restaurants, you'll probably have to make priority seating reservations. Of course, this will have an impact on your touring schedule.
Are you staying for Illuminations? If you are staying and it is during one of the busier times of the year, you'll want to scope out a good spot up to an hour ahead of the show.
You also really need to consider the pace at which you would like to tour and the types of things you want to experience. If you want to experience more of the traditional Disney-type attractions, you will need to spend a little more time in FW than otherwise. Alternatively, you could spend a LOT of time in WS just wandering around the countries, going through the shops, seeing the street performances, etc.
Good Luck. Have a great trip!