A straight 925 when compared against a M813 comes out on top in my opinion. The only advantage the M813 might have is it's a manual transmission (if you prefer a manual), and some parts may (just guessing on parts availability) be more readily available. I have found a source for anything I need for my 813, and anything I anticipate I'll need. I haven't looked for a 939 truck, but others have stated here that some parts are hard to come by. Also, you'll probably need a cdl or air brake endorsment for the 925, whereas you can get by without it on an 813. Opinions vary on that, so check your state laws.
They have the same engine, but the 925 has an automatic transmission. I understand the 925 has a faster road speed. The 925 is a bit roomier in the cab. The 925 has airbrakes as compared to the 813's air over hydraulic. The 925 has an air shift front axle vs. the sprag on the 813. Also, the 925 is probably going to be newer. In terms of weight and dimensions they are comparable with the 925 weighing a little more than a 813w/w.
That's all my 5 ton rookie opinion, take it for what it's worth. Many here have extensive experiance with both trucks. I own an 813, but I've only "owned" a 939 truck in Army service and havent had much hands on one at all. As a hobby truck the 813 is sweet. The 925s were terribly expensive when I bought my 813, but the prices have come down in some areas quite a bit.
Which M925 series were you looking at? A M925A1 has super singles, and a M925A2 has some significant differences over an A0/A1.