These 2 effects in Flex accomplish the same basic task; fading something in or out. The difference is, Fade utilizes a true alpha fade; meaning the alpha value of whatever object you tell it to fade will gradually change it’s alpha setting.
The cons are, this is very processor intensive, and device text, what all of Flex controls use by default, does not fade by changing it, or its parent’s alpha value. The pros are that for some designs, this looks very cool, and is a must for fading certain elements out without affecting other areas of the design unecessarely.
Dissolve on the other hand simply draws a fill over the object, defaulting to white, and fades that rect. The cons are, this looks pretty bad where you only want one object or irregulary shaped area of your design to fade since you can see the fill as a box when fading in our out in some transparent design situations. The pros are, if you use the default containers and do not use transparent backgrounds, this produces the same type of fade as a Fade, only less CPU intensive. Additionally, it allows device text to fade in and out since it’s a cover that is fading over top of the text, not the text itself.
For most Flex applications, a Dissolve is usually more appropriate to use vs. a Fade.
Hope those at MAX are having a fun time!