Most silicones are OK to use, A lot (includeing the one that I use) say not suitable for tanks for one reason only, and thats because the testing hasnt been done (I confirmed this today with my suppliers). I have used Dowl Corning 480 black for all my tanks that Im building at the moment (25 in total) with out any problems but remember to leave plenty of time for them to set properly (I give 2 weeks)
The cost of silicone should be no more than $10 but 480 or 580 should be able less than that.
Is one better than others, I dont really know but I use the ones that are use in high rise buildings and have confidence in them
Hope this helps a bit