I am need of some advice about about a product/service that i have recently developed.

I came up with an idea for a piece of software/web based service a few months ago and have come up with a working prototype. I have had a meeting with a patent attourney and last week submitted a patent application on the idea.
I am now spending the next few weeks producing a full version of the software/service from the prototype and hope to "go live" in the next month or so.

I am trying to work out the best way to allow people to use the service. I believe that I have two options and am unsure which is the best route to go down :

1) I charge users per each time they use the service. A user purchases say 10 credits for £1.00. Each time they use the service, they spend a credit. The cost is pretty cheap and shouldnt stop people from using the service.

2) I dont charge users for the service at all. I allow them to use it free of charge - and this will hopefully increase the number of people using the service. This would allow me to get a good number of people using the service (and testing it thoroughly in initial stages) . I would then be able to sell advertising space to companies on my service or the web site.

I am unsure as to the best route to go down.

The day to day runnings costs of my service are minimal - so i dont really need to charge for the service. My costs are the electricity for having a PC running 24 hours a day and having a yearly web hosting service (£40), plus the usual web site name fee's etc.

Obviously, i would like to have some income from my idea but as i said it isnt essential.

Can anyone give me any advice as to how much approximately could be made from advertising compared to charging users?
For example, if 10000 people looked at my web site - what revenue would be made from an advert appearing on this site? Or if an advert was included on 10000 emails being sent out from my service - what is an estimated income from this?

Also, for the initial few months, i believe that whatever route I choose (i.e. users pay, or advertising space) - I am best to allow people to use the service free of charge to build up the number of users of the service and to spread the word. Would this seem reasonable?

Any help in any of these matters would be greatly appreciated!