Comfort level is a big one, if you are the primary developer, and you like Entity Framework then that is a very big reason to stick with it. However, in your case, it sounds like you are coming in to a new environment that is using netTiers, you prefer EF, and you want an excuse to switch to your favorite framework.
I have used both frameworks, and I started using both of them about the same time. In my opinion they both have areas they shine in.
EF is better for small projects, it is primarily used for a code first approach and shines in allowing the application to upgrade the database as new versions of the app are rolled out. However, it also promotes poor sql query generation using linq-to-sql and consequently tends to have a lower overall data throughput then netTiers. If the developer has little direct database experience, or has no direct management access to the database, then EF will likely be a more attractive option for them as it will allow them to take back a little control.
netTiers shines in reducing code writing and maintenance. Unlike EF it only supports a database first approach. netTiers auto generates the entire DAL for you, and keeps it updated at the click of a generation button. It works better for larger projects, particularly web projects where you have full control over the hosted database and can easily push upgrades to it. netTiers Achilles heel is the CodeSmith config used to generate the DAL. This config probably needs to be kept in source control, because if it is ever lost and was highly customized, then it can be very difficult to re-create that so your DAL is generated the same way the next time you push the button (this can be an issue during developer turnover). netTiers also allows you to see all the DAL code, and debug as needed, whereas EF is just a dll you are stuck with.
Historically, netTiers was developed before EF really became a viable framework. It was developed to solve an issue that really had not yet been addressed. Since that time EF has really grown up and in many areas has surpassed netTiers, which has caused netTiers popularity to plummet. EF is far more configurable and flexible than netTiers. However, EF has never been able to touch netTiers in the area of code generation, and takes a lot more work to ensure it stays on par with netTiers in data throughput.
I have seen apps where developers have manually modified DAL code, which breaks the power of netTiers to reduce coding time by auto generating the DAL. As long as you are not stuck in that situation and you already have a solid netTiers setup, then trying to rip it out and convert over to EF would probably be a waste of effort.
