Go with the Dac. You never get your money back or break even when selling an upgraded unit.
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It also depends on what you plan to have as sources.
The opamp upgrade to the CD25 is cheap, very easy to do and makes a dramatic difference. If CD's are going to be your source for digital music the upgrade gives you unbeatable bang for the buck.
If you think you'll be using other sources like a music server or streaming audio then a DAC might give you the same audio quality but spread over multiple inputs.