As with most receiver manufacturers, when you go up the model line you get incremental additional features for a large increase in price. In this case, the prices are similar because both models have been out for awhile. It shows you how quickly prices on electronics drop.
The 7xx series is where Onkyo adds pre-outs for each of the channels, IR in, Multi-EQ speaker setup and sometimes THX certification. The power increase is marginal and will not be noticeable.
If you won't be adding an additional amp and are fine with the speaker setup using only 2 or 3 postions, then you don't need the 7xx series. Every year Onkyo migrates features from their higher number receivers down into the lower models. At this point, the 607 has everything one would reasonably need.