the ROI is based on hands-on value. Whether the internal price is higher or lower than the external price (exchanges).
Taking the external price isn't possible due to the extreme fluctuations.
With that said: all promotion services take the same data, so regardless of which one you take, the negative downside would be the same. However, with , you're on the more safer side.
Hope this could answer your question.
RE: Flash Sale 6th September: 15% ROI for @smartmarket