Hi Aron,
This is a matter of dilution in your title, I think that density of the title still matters but not when talking about preposition, if my title stays in the 70 words limit I'll stick with the grammar proof title but if you need to save space a preposition may go out. Consider that you may want to rank high for both the keyword with and without the preposition, however my assumption is for new sites when you have to kick off the site.
If you're already ranking in top3 I won't risk lowering my rankings by changing the title tag, also because tweak the title without any other effective change in the page would give google signals of seo tactics which won't help you at all.
If you're ranking good then let the title as it is. Winning team doesn't change 