Alex,
The two aren't mutually exclusive. We find the best success is actually a synergy between an agencies talents and a third party solution.
For example, BestBuy.com and Target.com both use Amazon.com's architecture-- because Amazon.com is a rockstar platform and they have designers to create an identity and presence that's uniquely theirs.
When you approach it from that perspective there honestly isn't that much of a price difference. Obviously having an agency build one from scratch is great but will be more expensive than is justified for a smaller dealership. Going with strictly templates ensures you'll have a decent site-- that's unfortunately just like all the other decent sites that exist right now.
Ultimately, it's all about conversions. Most sites from third parties convert at least 5%, many up to 12%.