You can find people who have owned one or the other over on head-fi.org. I can't think of anyone who has owned *both* though, but I might be mistaken.
You may already have considered the following, but for anyone else who is reading along:
The Grace is does not require drivers, whereas I the Mini-DAC requires that you use Apogee's drivers. Some people have different takes on this, so I leave it to you whether or not this is pro/con. Both should function fine under either Mac OS X or recent versions of Windows.
The Mini-DAC should eventually have a Firewire option card (it would replace the USB card), there is some discussion about it on gearslutz.com - this may or may not be of interest to you. Firewire can handle higher sampling rates than USB (192KHz, vs. 48 KHz)
The Mini-DAC has XLR balanced outputs AND a stereo mini-jack. You could get adapters for either output to allow you to use RCA jacks. Whereas the Grace has only unbalanced RCA outputs. The point being I guess, that if you really want balanced outputs then the Mini-DAC might have the edge.
The Mini-DAC is also at least half the price of the m902, though the m902 has a remote option. Either get a compat/universal one that works with their codes or buy their $150 (US) option.
The discussion over at head-fi.org right now sounds like the DAC in the m902 may not be as great as you'd hope. So unless you have other reasons to favour one over the other - it might not be worth paying twice as much to get the m902, you could instead invest that in other aspects of your audio system, speakers, amp, etc.
