We have run a bunch of cars with E85 and water/meth now, and I tend to like water/meth setups for a couple reasons.
From our experience you can get very similar gains from E85 or from water/meth setups, usually around 10-15% wheel hp increase. Both of them allow you to run more timing without detonation which is where the power gain is from, but they do it in different ways.
The reason I like the water/meth personally is that it is very easy to find windshield washer fluid whenever I fill up with gas, but with E85 I have to look for certain stations which can be a pain when traveling. Also since E85 requires you to deliver more fuel for the same power, you run higher duty cycle on the injectors (this is why you have to upgrade the injectors with the E85 tune). Also, E85 will get worse gas mileage then regular gas since it has lower energy density than regular gasoline. With the water/meth you only use it when you go full throttle, so your fuel economy stays the same, you will make about the same power as E85 and it keeps your engine really clean and carbon free which also helps with preventing detonation. On Direct injection cars we really like it also as it keeps the intake valves clean.
So that is why we like water/meth injection over E85. Hope that helps.
Dave