I would buy the branded one from a reputable source such as an ICOM dealer in the UK - it is after all the sole means by which your radio radiates RF energy. Handhelds are weaker, less efficient transmitters by nature so you want to give your safety device the best chance possible.
Also - when you've bought it do a radio check with a mate on another boat, preferably at increasing distances from each other to give YOU confidence that your radio is back up to scratch.
