Postgres performance comments from a MySQL user

Some backs-story. I'm in the process of converting our internal file based
data storage to an RDBMS. After looking a bit at PostgreSQL and MySQL, I
chose Postgresql. My boss has heard of MySQL and has not heard of
PostgreSQL and every now and then ahe make allusions that we shuold be
using MySQL.

One comment he got from the architect of another web site is as follows

> If we were to start again
> from scratch now, I'd still use InnoDB over postgres unless the
> performance picked up with postgres recently.
>
> Keep in mind our application is very write-heavy so your numbers may
> be different. Does postgres still keep the old row versions in the
> primary-key B-Tree? If it does I doubt performance improved much for
> write-heavy apps, that was a very poor design decision by them. InnoDB
> takes the Oracle route of moving old row versions to a seperate
> on-disk data structure.

Does what he say make sense? If so, has the situation changed? BNasically,
I need something intelligent to say to my boss to either counter or
mitigate his perception.

Thanks

Take care,
Jay