You’re right to question that; 30” is better. This salesperson isn’t designing your kitchen, is he? I hope not.

Post a scale drawing of your floorplan and some photos. That way, you'll get much better feedback. Also, this forum generally frowns on cooktops in islands. Most of the time, it's just not very practical.

For example, even with induction, which has less stringent requirements than gas, the vent hood will be much more difficult to get to work than for a wall installation.

Also, safety is problematic, especially when the island is on the smaller side.

I would never want a cooktop on an island. Venting is the reason why. Plus, like mentioned, you need a good chunk of space for prepping. I have a 36" against the wall with my wall oven a foot or so to the left. I do all my prepping on the right side which has adequate room and the sink is also right there.

I'd rethink your design.

When I remodeled my kitchen, I learned that the minimum width of countertop adjacent to a range should be 15". That's what I did (I didn't have much choice as the kitchen is small) but that space isn't very useful. I'd go with the 30" cooktop.

I have an aisle kitchen with the range on one side and the "prep space" plus sink on the other. I don't like having to turn around between the prep side and range side.

I'd never put a cooktop on the island, unless there really was no other place to put it. I'm sure there is. For me, a 36 inch cooktop is a necessity.

I think your initial plan sounds better. By all means post drawings!

There are other reasons nbt to put a cooktop in an island, but if it is an induction cooktop the 36 inch IMO has a lot of inherent landing space on its surface unless you are using all hobs at once. The most I have ever had going is 3 at holiday time and that still leaves 2 empty hobs for landing space. Love that smooth top and the instant off feature!

I had put a 30 inch glass (radiant)cooktop into my kitchen in my island in my primary residence. I found that I only had one large ring to cook on, 1 medium and 2 small, so that did not give me the 2 large rings I really would have preferred. . At the time I did this, there wasn't as wide a range of choices. If you do go with a 30 inch, make sure there are 2 large rings.

Now that I am redoing the kitchen in my vacation home, I find I don't stand as much at the cooktop as I did (or thought I did )18 years ago so don't need it facing into the room and prefer the option of an overhead vent hood venting out an exterior wall. Do an imaginary walk through of your space preparing some meals and snacks and see how you work best and how long you actually spend at the cooktop. If you do an overhead hood, how will that affect your sight lines and will it make you feel more closed in. I have a downdraft with my cooktop and it really is insufficient with the big pots on the front large element.

I have a 36 inch cooktop in my island that is 58 inches wide, house was originally built that way, even with 6 more inches I wouldn't do it again. I am a social cook that enjoys facing my family like you, but the lack of space surrounding is a safety issue with my young kids. I personally would need a 8 foot island to put that size of cooktop on. I'm planning a remodel soon to fix.