The key to getting the most out of your performance on the bike is to even out your effort, eliminating big spikes and drops in your power over the entire course. While heart rate can fluctuate based on a number of factors that can sometimes be unreliable, measuring your output based on watts is likely your best option.
All power meters were once near the $2,000 range, but now there are plenty of affordable models under $1,000. While it's hard to determine how much time you'll save (this depends at how good or bad you are at pacing yourself), a power meter can't be understated. With practice, you can perfect your pacing and give an even effort that you know you can maintain for the duration of the race—which can play a huge factor in your run performance.