This is a great question, and different scenarios will produce different solutions. I think the simplest way to answer this is to counter with a different question: Why not BOTH? Even though the internet is the looming future of content sharing, there is still an audience on both sides of this fence. Some will overlap, but the truth is, some people don’t care for surfing the web, just as some don’t care for watching commercials on TV. If you want to reach both groups, maybe TV and web video working in tandem is ideal for your marketing strategy.
Here is an idea: Produce a solid web video or series of web videos that share the message you have for your audience. The web video can be slightly longer (but not too long), because when it’s living on your website, the airtime is free. Once this is complete, ask the producer to use existing footage to cut a 30-second spot and a 15-second spot to be aired on local TV. Since the footage was already shot for the web video, it should be a fairly simple process to create the spots. Additional shooting should not be necessary.