Hmm, again it's a case of apples and oranges. I don't have both rxs - I do have a RX320, and do a fair amount of utility listening - so my opinion may not necessarily be unbiased. However, the edge might go to the R75 IF you consider that the DSP is already there. You CAN add DSP audio processing with a program for the RX320. So the edge is, in my opinion, somewhat minor....
On the other hand, the RX320 does keep its edge in the selectivity department. With 32 filters (depending on the software, of course) from 8 khz down to 300 hz, you won't find many problems in that department. It becomes even more important in the marine bands, where, when propagation is good, things can get very crowded in a hurry. While the R75 is also very good here, you simply don't have the flexibililty that you do with the 320.
An interesting fact here - RXPlus is a program that works with BOTH radios. So you CAN get audio processing for BOTH radios from the SAME program. However, the 12khz audio processor is available on the 320, but not on the R75 (without modifications, that is...)
For utility DXing, both have their strengths and weaknesses. Some folks prefer having a hard knob in front of them with what they perceive as a 'real radio'. Others like the additional flexibility that comes with DSP processing....
I should note here that, for purposes of definition, both radios are DSP DRIVEN, but not fully DSP; this is because the audio chain in both radios is analog. This leaves a great deal of room for add ons, some of which are free (SR5, for example - the link for that is available on the AR5000 wiki under 'AR5 Software') and some of which have some cost involved.
Tell you what - since you're interested in utilities, why not let Ute listeners tell you their preferences? If you joined the R75 and RX320 Yahoo groups, and the WUN list at QTH net, you would get just such information. Certainly more valuable then some of the ramblings I've presented....73s and GL...Mike