I want to correct a couple statements on this thread that may mislead others:
...Obviously the splitter is the easy route but the fact that the VHF antenna is "tuned" to the radio and not best optimized for the AIS and the fact that hey share makes me wonder if this the best solution...
AIS transmits over VHF channels 87B and 88B. It uses the same design VHF antenna as any VHF radio. There is no special tuning of the antenna.
...If I were you I would buy the Standard Horizon unit and sell my Icom unit...
In general, DSC radios can only be sold with your boat. To activate a DSC radio (with our without AIS), you must register a MMSI for your vessel, and then enter the MMSI into the radio's permanent flash ROM. Once you have done that, the is radio permanently paired with the vessel. You can pass the MMSI to another owner (when you sell the vessel), but you cannot move the MMSI to another boat, and you cannot erase or change the MMSI once it's entered into the radio.
It is important that owners of DSC radios do not assume that they can resell the radios to offset the cost of purchasing a new radio. It is doubly important that potential purchasers of used DSC radios not get duped into buying a used radio that is permanently registered to someone else's vessel.
My advice is to NEVER buy a second-hand DSC radio (fixed mount or handheld), otherwise you won't be able to register it. Non-DSC handhelds are OK to buy second-hand (since there's no MMSI registration). Since DSC has been a requirement for all fixed-mount radios made for the last 10+ years, non-DSC fixed mount radios are all too obsolete to be worth buying on the used market.