Did yu get an answer to this? I'm sure i have seen islands with both those things. Don;t know any rules but it would be annoying to have the sink right up against any stovetop. I would leave at least 15cm

Did you get an answer, if not ask your electrician he should have a standards book.

I think of the two a stove top is better on the island.

See This Article for the reasons why.

One of the issues with cooktops in the island is that you should have a range hood above - and pending on your ceiling design this may not be achievable or wanted. Also note that Australian standard states that all cook tops need an isolator switch within 1 meter of the cook top (normally I would put it as part of a double GPO on the splash back close to the cooktop - this can be problematic with an island.) I like to put prep bowls in the islands, but generally leave all main sinks and cooktops in the main kitchen bench, having them with a splash back behind is a lot more practical when it comes to cleaning.