I personally use Omega One because it's what we use at work and I have noticed good results, though I am not accustomed to using Tetramin as I'm not sure I've ever tried it. I'm sure there's nothing wrong with it whatsoever in any real capacity. However, I am a label shopper and try to go with what at least seems to have more nutrients and less fillers. Based on these comparisons of ingredient lists I'd be more inclined to go with the Omega if I had to choose. But I would not freak out if all I had access to was the Tetramin.. to each their own. 
To be fair, they both use the same preservatives and the preservative in question apparently has its own little bout of controversy.
http://www.epa.gov/oppsrrd1/REDs/factsheets/0003fact.pdf
Tetramin:
Fish meal, dried yeast, ground brown rice, shrimp meal, wheat gluten, potato protein, feeding oat meal, dehulled soybean meal, fish oil, soybean oil, algae meal, sorbitol, lecithin, ascorbic acid (source of vitamin C), inositol, niacin, L-ascorbyl-2-polyphosphate, riboflavin-5-phosphate, A-tocopherol-acetate (source of vitamin E), D-calcium pentothenate, thiamine mononitrate, pyridoxine hydrochloride, vitamin A palmitate, menadione sodium bisulfate complex, cyanocobalamin, cholecalciferol, and colors blue 2 lake, red 3 dye, yellow 5 lake, yellow 6 lake, ethoxyquin (as a preservative).
Omega One:
Whole Salmon, Whole Herring, Black Cod, Halibut, Whole Shrimp, Whole Krill, Wheat Flour, Wheat Gluten, Fresh Kelp, Lecithin, Astaxanthin, L-Ascorbyl-2-Phosphate (Source of Vitamin C),
Natural and Artificial Colors, Vitamin A Acetate, Vitamin D3 Supplement, Vitamin E Supplement, Vitamin B12 Supplement, Riboflavin, Niacin, Pantothenic Acid, Folic Acid, Biotin, Inositol, Tocopherol (Preservative), Ethoxyquin (Preservative).
To be fair, they both use the same preservatives and the preservative in question apparently has its own little bout of controversy.
http://www.epa.gov/oppsrrd1/REDs/factsheets/0003fact.pdf
Tetramin:
Fish meal, dried yeast, ground brown rice, shrimp meal, wheat gluten, potato protein, feeding oat meal, dehulled soybean meal, fish oil, soybean oil, algae meal, sorbitol, lecithin, ascorbic acid (source of vitamin C), inositol, niacin, L-ascorbyl-2-polyphosphate, riboflavin-5-phosphate, A-tocopherol-acetate (source of vitamin E), D-calcium pentothenate, thiamine mononitrate, pyridoxine hydrochloride, vitamin A palmitate, menadione sodium bisulfate complex, cyanocobalamin, cholecalciferol, and colors blue 2 lake, red 3 dye, yellow 5 lake, yellow 6 lake, ethoxyquin (as a preservative).
Omega One:
Whole Salmon, Whole Herring, Black Cod, Halibut, Whole Shrimp, Whole Krill, Wheat Flour, Wheat Gluten, Fresh Kelp, Lecithin, Astaxanthin, L-Ascorbyl-2-Phosphate (Source of Vitamin C),
Natural and Artificial Colors, Vitamin A Acetate, Vitamin D3 Supplement, Vitamin E Supplement, Vitamin B12 Supplement, Riboflavin, Niacin, Pantothenic Acid, Folic Acid, Biotin, Inositol, Tocopherol (Preservative), Ethoxyquin (Preservative).
