The final result of your mastering at the present time is going to be a CD which is 44.1 KHz with 16 bits of resolution.

The best file to send at the present time would be 44.1KHz and 24 bits.

Dithering from 24 bit to 16 bit works well and is not a problem. Down sampling from 48 KHz to 44.1 can present some problems.

If this is going to be for release sometime in the future on some other high res format then you may want to think doing it at a higher sampling rate and bit length for your overall master.

I just mastered a blues project that was 32 bit and 48 Khz. It came out fine and the client was well pleased. I also just did a session where the incoming was 44.1kHz at 16 bit and the client was also pleased.

Higher sampling rates and bit length don't do much for material with a dynamic range of less than 15 dB and a signal to noise ratio of 50 dB or less. Where you really hear the difference is on acoustic music that has a wide dynamic range done in a good quiet venue like classical or jazz.

My advice would be to ask the mastering engineer that is going to do your stuff what he or she prefers.

Hope this helps.

I always preach to use the highest quality available to you. 44.1 is good but 48K is better and 96K is even better. You as a mix engineer should use the highest quality. Don't worry about the conversion down to 44.1. Most pro mastering studios will go through a quality DA, do the eq.comp. in the anaolog domain and then through a quality AD into the workstation. If you want to stay digital make sure the mastering house has very high quality converters. The end result will be noticebally better. These are the little things that make a difference in the project.
Remember you may be releasing it on 16bit PMC digital now. But who knows what you may release it on in the future.

I use Prism Dream AD/DA's. We have others here as well but I have never heard anything sound as good as the Prism. And it makes a difference.