"It would be nice if the ETTL could be turned off on the ST-E2."
Why would you need to turn the ETTL off? If you are using the ST E2 on it's own then the camera will expose for the conditions. The ETTL works by firing a preflash to measure the light levels, if you don't have a flash then it won't be able to do a preflash!
Canons ST E2 is expensive and not particularly good especially in bright sunlight, it uses an odd and very expensive battery which doesn't last long, make sure you have a few spares for just in case. The Yongnuo unit is better in almost every way - it uses AA batteries and has a much better range, it works in bright sunshine. Unfortunately it has a weakness in the grid it projects for the autofocus is poor and although it will focus , it's a bit of a hit & miss affair. Using it as a commander for the Canon flash system is a much better choice than the OEM version.