I've been tracking some of the major activites and events happening in and around DLR on the
2015 Calendar of Events & School Breaks and DLR thread.
In terms of the two choices in time, I think it really comes down to crowds, park hours, entertainment, and decor. As
SMD mentioned, if you go in late August, the parks will likely be still operating (to some degree) on the summer schedule. This means generally longer hours and more entertainment, but also potentially more crowds. The summer SoCal and SoCal Select AP Blockout ends on August 17th, so I would expect to see an uptick in crowds at that point (especially on the weekend) as folks return to the parks for the first time since June. Last year, fireworks ran nightly through Labor Day, but F! did go down to Fri-Sun only schedule the week before Labor Day. With Labor Day being late this year (9/7), it's not clear when the summer schedule will start to taper down. Also, it's worth noting that Haunted Mansion usually closes towards the end of August so that they can install the Nightmare Before Christmas Overlay (I'm guess it'll be August 24 this year).
Late September is a fun time; the park will be decorated for HalloweenTime! If you do decide to go during HalloweenTime, I highly recommend joining us on the
HalloweenTime SuperThread. We don't know quite what the schedule will be this year because based on historical calendars, HalloweenTime would normally start on September 11th, but given the signifcance of that day, it seems unlikely that DLR will opt to start a festive season on the day. So I'm guessing HalloweenTime will start on Saturday, 9/12. However, it could start as late as 9/18 (the next Friday). The first MIckey's Halloween Party has usually been on the last Friday in September, but there are rumors that the parties may be starting earlier and/or there may be more of them. There are also rumors that the parties might move to DCA, so at this point, we really don't know much about what is going to be happening during HalloweenTime. September weekends will be busy once HalloweenTime starts as guests come to see the Halloween overlays on HM and Space, along with the decor and any other Halloween-themed activities. Weekdays (Mon-Thu) are usually pretty managaable, but have short park operating hours.
So whether one month is better than the other really depends on what you are looking for. If it's strictly based on crowd levels, I would look to go towards the end of September, but avoid the weekends if possible.