- How many days will we visit the parks?
- Do we want to park-hop?
- Will we visit the water parks or DisneyQuest?
- Will we be returning to WDW for another trip in the future?
With Magic Your Way tickets, you can add the Park Hopper or Water Parks & More features even after purchase for a flat fee. You can also add the No Expiration feature after purchase, but this feature gets exponentially higher the more days that are on the ticket (and even if you've used all by one day on the ticket, you'll still pay the full upgrade fee).
So...if you're fairly certain you'll be returning to the World in the next few years, you might consider a multi-day ticket (twice the length of your intended stay for the current trip) with No Expiration added. Ticket prices tend to increase every year, so this is almost always the best deal for return visitors. (And it's always a bonus on your next visit when you already have that expense covered!) But, if you think you'll be returning to WDW within the same calendar year, an Annual Pass might actually be a cheaper ticket for you. Same goes if you're planning to stay more than, say, 10 days. (In addition, there are many "perks" for Annual Passholders.)
You can check out all the WDW ticket options here, and you can find discounts on many of these options here. Be sure to do your research--and the math!--before you purchase tickets.