Lisbon!
While Amsterdam would probably not be a bad choice (easy to get to from just about anywhere thanks to Schiphol Airport, plenty of big conference venues and things to do around the conference) I'd prefer Lisbon simply because of the climate.
The upside is also that it gives some flexibility planning-wise. You could easily do it as early as the end of February or as late as October and still have nice weather. Quite useful in an already crowded conference calendar.