Other than the first link actually being for the GE product, which hasn't actually been manufactured by GE for some time (potentially way out of date), the only difference I am seeing is the working time. I have only used Momentive RTV103 (black), which states a working time of 10 minutes. The second link states a hardening time of 20 minutes, which may be double that of the working time. I have no idea though. The 12C states a 30 minute working time. I think this might be too long for the size tanks that I have biult, but if you are building a larger tank, this may be more suitable. The 10 minute working time may be too short for some tank building methods. My concern would be that the longer working time would lead to the possibility of things shifting too much between assembly and the RTV setting up. With the 10 minute working time of the 103 and the smaller tank sizes that I have built, I found that the silicone holds everything in place immediately. I don't know if there would be a difference with the 12C.
You can always call up the manufacturer. From what I have read on other forums, they were quite willing to discuss application issues and answer questions for hobbiests.
I usually feel kind of guilty using the quick reply... my replies are rarely quick.
Last edited by Beer; 05-14-2014 at 03:04 AM.