DAC Chips

Both chips in PRO version are 2x8 channels.  My Benchmark DAC3 sums them by 4 and uses dual differential outputs.  That way they use all 16 channels.  9038PRO according to links provided by George, has dynamic range of 140dB, but it is in mono mode (8 summing?), while for 129dB in 9028PRO mode is not specified.  They differ very little in THD + Noise (-122dB vs -120dB) and that's perhaps why Benchmark chose cheaper 9028PRO for DAC3.  

My Ayre uses an ESS DAC chip also.

I also think that what @goofyfoot said is an important point. It is as silly to judge a DAC by its chipset as by the thickness of the front panel.

The 9038 Pro chips can be ran in a high current mode.  This allows higher output voltage/current from the DAC chip itself.  The downside is heat (which requires heatsinks on the DAC chips themselves) as well as an I/V stage that is capable of the higher voltage swings.  I know of only two manufacturers that support the "high current" mode (LKS and Audio-GD).  If you see the 9038/9028 in equipment without heatsinks or using normal op amps for I/V, then they are running them in standard mode, which isn't really any different from the previous 9018 generation DAC chips.