To me there is no question that the UE700 is going to sound better. For one it has a much flatter frequency response making it produce exactly what you are listening to more accurately. The S4i has a +15dB bump from bass to sub-bass ( 90hz ) which is simply terrible. This is going to add bloat, sluggishness, and boom to the low end. There is no doubt that the S4i is a bass heavy boomer.
The UE700 have their issues. The first and most important is the build quality. I avoid the UE700 because they are known for falling apart at the slightest tug of the cable. I would not risk purchasing an IEM over $100 unless it was built like a tank. We carry them around, toss them into a book bag, or snag them on a random piece of equipment. We should not have to worry about the sudden destruction of a an expensive IEM from normal day to day use.
Take a look at the impedance curve for the UE700 vs the S4i. See how the UE700 starts to wave around and show some dependence upon frequency after 1kHz? This can be an indicator that the drivers are going to be hard to control. They may still sound nice, but if the impedance is moving around on you your amp / iPod is going to have to work that much harder to control the drivers.
The UE700 do isolate better than the S4i which is a plus.
For the 500Hz square wave response you see that the S4i dips below 0 dB indicating some control issues.
Finally the 50Hz ( bass frequencies ) test show that the UE700 reproduce the 50Hz square wave much more accurately than the S4i. See how the curve for the UE700 remains very straight and not curved? This is a good sign of control in the bass region. The S4i curves up showing it is going to have some issues at low frequencies say sub 40Hz.
If you are looking for a compromise as well as a well made IEM I would go for something like the Brainwavz M2.
They do not have the build issues of the UE700 nor the sound issues of the S4i. I highly recommend them especially if you want clear bass and really clear and up-front mids. As with most IEM the treble rolls off, but not as much for the M2 as many other IEM like the Monster TurbinePro Copper seen below.