From a pure economics point of view renting your primary light makes sense. From an experience point of view, rental lights are often "iffy", and batteries are not often fresh, so your entire night diving experience can hang in the balance of these critical items.
My recommendation would be for you to consider renting your primary light, but owning a good quality back-up light, with fresh batteries, that may in the end save your dive! Plus you have the added benefit of a handy light that is small enough to take on daytime dives to peer under ledges, in holes, etc, and improve your dive experience. A good back-up light can be had for $20-$40, and many, such as Princeton, Pelican or Tektite, come with a lifetime warranty.
Hope this helps!