Incidental is not the same as continuous food contact, which RTV108/103 is rated for. 108/103 also do not carry warnings against aquarium use. The black (103) carries all of the same ratings as the clear (108).
I was not able to find a MSDS or technical data sheet on GE Silicone I, would you happen to know where to find that information? 108/103 series is readily available. I would like to compare the differences. What are the food contact codes and standards listed on the Silicone I? I don't have any to reference.
Silicone I can work, lots of people have not had issues, a lot have as well. Not everyone that has had issues had a tube of Silicone II. It is a consumer product not intended for structural applications and may not have the same QA standards. People have reported getting products that did not match what they had before in previous tubes of Silicone I. This could possibly be the issue people have had with Silicone I; the wrong product in the tube. Also Silicone I is not an adhesive, unlike 108/103, it is a sealant. The bond strength is lower. I really need to see the technical data sheet to know what the difference is, but my initial search came up empty handed (including searching the manufacturer's site).
It is your choice, so go with what you feel comfortable using. The cost difference is pretty minimal, especially compared to the life that is potentially going in your tank and the possible damage that my result from failure. Personally I do not see the $4-$5 saved as being worth the risk. Silicone I has worked fine for a lot of people, but not all. The only reports I have seen (two or three) that had issues with 103/108 were due to construction issues that were readily admitted by the people that built the tanks.
If you want to find out a bit more, search ReefCentral. UncleOf6 on there has a lot of good information regarding silicone and tank building. He builds a lot of tanks and has spent a lot of time speaking directly with the application engineers of several silicone manufacturers over the past few years regarding different products.
I usually feel kind of guilty using the quick reply... my replies are rarely quick.