There is no one model which works "best" for all applications, if there were then that would be the only model supported 
For well established titles or titles that are able to get their own exposure (ex Built For BlackBerry titles, featured apps...) then a paid app may make sense. For lesser known titles, especially once the # of apps gets much higher, exposure is *much* easier with a free app. Keep in mind that to make the "Top Paid Apps" list your app would need to be able to be sold well under its own brand, if it can't it won't make the list and the exposure will not be there.
A free + paid model has tons of drawbacks: User upgrades are far less likely in most scenarios, users typically lose all data/progress when updating, social integration (BBM, Scoreloop) does not work between the 2 versions etc etc.
A freemium model works very well for lesser known apps that want to provide a good user experience, get good exposure, then up-sell a percentage of users on digital goods. Again this is not for everyone but works very well for some people/apps. I personally prefer this model when implemented properly unless I am already familiar with the brand in which case I will pay for the app upfront.
