I think your subject line raises one question, but the body of your question raises many more! Not that that’s a problem!
I agree in general with the other comments here, but as always answers are highly dependent on your particular situation. I believe it is still possible to make money on ads if you’re in the right big-money niche, but it is very, very hard. I don’t know what vertical your directory is targeting, so you’d know best. In most situations though, the common wisdom is that you shouldn’t aim to make much money through ads these days, so I would encourage you to put more emphasis on the freemium tiers you mentioned.
Without your tool, how difficult is it to access the information you’re providing in your directory? If you’re providing actual value to your users (particularly if they are businesses), you should charge for it. You’ll be much more likely to develop a viable revenue stream.
I’d rather have more eyes that paying eyes
Why is this? I don’t necessarily disagree, but I’m curious to why you think this. It costs money to run your business, and your time is very valuable — in money and/or in opportunity cost. Unless you’ve got a substantial outside investment or a big pile of your own cash, you’re going to care about the paying eyes sooner rather than later, if not just to keep the doors open and your attention on the project.
As to your main question, I think getting their email address is critical, but if you can give them some value up front before you require the email, you’ll probably increase your conversion rate. Can they see a sample or overview of the results, and then input their email address to get more results (feels a bit to Experts Exchange to me)? Maybe you can provide full access to the results, and offer to provide live updates on their search if they enter their email address? This last option feels the less sales-y to me, but that doesn’t make it the right decision for your project.
In general, if most of your users are not going to pay you directly, you’re going to need to get them to view many ads — which means either visiting your site many times or sending them many newsletters — in order to break even on the cost of acquiring each visitor. Ultimately, I think that is why you need to figure out a way to at least get their email address.
Good luck, whichever way you go!