Yes, he must be somewhere other than the US or Canada (the last "new" 2 stroke Merc was sold in 2012).
The Super 15 is nothing more than a detuned 18 horse Tohatsu. I am unaware of their method of "detuning" (could be restricter plate, jets, restricted exhaust port? - never had the opportunity to tear one down for a look).
Because of that it will have greater displacement (294 cc/17.9 cube) than the 15 Yami (246 cc/15 cube). That will translate into better torque, particularly at the lower end (with the super 15).
Since they are both very much in production, at least in the Asian/Australian market, parts would not be an issue.
The Super 15 has been in production for about 10 years or so and the 18 Tohatsu, the (parent) has been around for 30 years - so not a "flash in the pan" model by any means.
For me it would boil down to "price" and "warranty" - they are only 15's after all, you are not going to water ski behind either of them.