Preferred Audio Output, USB or 3.5mm?

HDMI has higher bandwidth than USB and transmits video. Firewire is on it's way out. You will not find a new processor with it. You may find it on a few DACs.

Hdmi is great for ht and multichannel applications, but I wouldn't use it for a 2 channel music setup. Some dacs use an hdmi connection for i2s, but that is different than your standard hdmi on a processor/receiver.

On The Mac, I would look into a USB to spdif converter which sounds better than the optical output in my experience. Even the inexpensive hiface sounds better than a glass mini toslink from my Mac mini.

I know most DACs that use HDMI for I2s only but they should use regular HDMI. HDMI will transmit SACD, DSD, DVDA, FLAC and every other HI REZ format I know of. USB is good but limited to what HI REZ formats it can transmit and so is SPDIF. Plus if you want to be able to use Bluray audio with your fancy DAC, SPIDF will not pass the uncompressed audio. This is the same for computers with Bluray drives.

I use the USB port of my mac to hook up my Amp/dac. Havent used HDMI yet.

Since the original topic is about 2 channel, I don't believe regular HDMI to be the better connection. It's just not necessary and it isn't a superior connection compared to other, less expensive to implement options. SPDIF, usb and firewire is fully capable of transmitting hi-res 2 channel formats. You can use a $40 hdmi audio de-embedder for SACD's and the uncompressed audio tracks on blu-rays and run the spdif output to your favorite dac if you must. You can rip the DVD-A's stereo tracks to your computer and play them through your dac as a file or burn them to a disc and play them through a regular dvd player.