The MX680 only has a straight jack on the short cord. Plug the extension in and then it is right angled. Though I like that since I use an iriver T9, wear it around my neck and can do any kind of sports/ball playing with very little cable and weight and still have easy access to the device.
That is one thing that they dropped the ball on. People complained and they took the short cord away in the new series. Problem was that people only really had a problem with the large and cumbersome volume control at the connection point and not so much the modular cable aspect. So, not sure if that is an issue or preference for you. Other than that the MX685 looks to be better as far as fit than the previous one and the sound doesn't seem much different. Though with buds better fit = better sound, so the 685 could sound better based on having a better fit.
Me, I have the PMX680 and would not get the PMX685i. The 685i is 3 grams heavier, is an i version which I don't need, and is long cable only.
If the price is the same, then decide based on the cable and and which type of fit mechanism you think would work better for you.