I've been to Nats "Winterfest" and Orioles "FanFest."
FanFest was, by far, the superior of the two experiences. I've talked to a lot of people who go to the other events by other teams (caravans, meet and greets, something with "fest" involved in the name) and the concensous has always been "I wish *team* would do their FanFest like Baltimore!"
The autographs, touring Camden Yards, the promo room, the Kids Autograph line, the different chats, the team store and the discounts during the day, the Minor League teams booths, the Authentics shop, the card and misc. dealers...It is awesome and there is something to do for everyone, no matter the age and level of fandom.