Hello, my friend.
I have been asking myself the same question you ask here 
Well, i have done very scientific statistical research on a corner of newspaper while i had my morning coffee
And the results I got weren't surprising at all. The conversion rates on traditional advertising are way lower and much more expensive.
Now, as to the question you asked. The only(!) way to do this properly is to run only one(!) type of advertising at the time, for somewhat significant period of time (i'd say at least three months, plus another three months to track "snowball" results).
So, this is the way I'd do it (requires lots of time, money and several clients who are ready to be lab hamsters 
- Let's say you have 4 non-seasonal-type-of-business clients, without any advertisement/marketing running at the moment. Track their conversion rates for three months, traffic spikes, growth over this time and all metrics like this.
- Then start only one type of marketing per client. Let's say first client gets internet marketing, second - TV, third - radio, fourth - billboards/outside display marketing. Let it run for three months, see how their metrics are relatively changing (or not changing). Also dont forget about post-advertisement snowball period (may another 2-3 months).
- Compare impacts and ROIs - now you gonna have somewhat "scientific" results.
While running all those experiments, it'd make sense to use promo urls for non-internet marketing channels, e.g. domain.com/nameofradio - this way you can promote your website with certain landing page and track visits to it.
P.S. Of course, three months is short period of time to see good results. Extend it to 6-12 months if it's possible
Also, if you do it on 4 different clients, there is gonna larger margin of error. So, in perfect world you'd do it on the same client 
Hope this helps and good luck 