You might not need transactions now, but what if you need then later? If the speed difference between postgress and MySQL performance (what is that, 10%? a boundary case ) is going to kill your app, then get a 10% faster server.
What if you don't need transactions in PG?
Let's not forget MySQL has engines like MEMORY, ARCHIVE, BLACKHOLE (not to mention third party engines).
I was trying to forget about MySQL and it's plethora of engines.