I have a 12-15 mile commute, so similar to yours. I use a road bike that was pretty darned expensive, but I justified it by saying I deserved it. Not sure that's true, but I have no regrets. I smile every mile on that thing. You really do get more for the money you spend, and commuting allows me to enjoy my investment every day.

So my recommendation? Buy something you're not sure you deserve, but you know you'll enjoy.