I actually just had a great experience with both of these websites on a recent flight from Toronto to Frankfurt. I checked Seat Guru to get an idea of the seating configuration and possible pros and cons of each seat. Then I used Aerolopa to get a better idea of which seats were available and which ones would be best for my preferences.

Thanks to the information I gathered from both sites, I was able to snag a window seat with a little extra legroom. The flight was long, but the seat was so comfortable that I was able to sleep for a good chunk of the flight.