If you are going with a truck with the Automatic, go with the 6.0L as the 5R110 transmission can hold more power over stock than the 4R100 used with 7.3Ls. If you are lucky and find a ZF6, that won't matter.
Depending on the 7.3L in question, you may find yourself limited to around 400RWHP with the 1st gen PMF rods of the later trucks. Also getting a 7.3L over 500RWHP takes considerably more investment than a 6.0L.
The 6.0L has its flaws, and most are solved with the aftermarket. Check my sigs for monitoring, bullet-proofing and performance capabilities with the 6.0L. You can get to around 440-450RWHP on electronic progamming (SCT with FICM programming) alone.
I'd take the 6.0L, if you want a considerable amount of power on hand. Also with an '05-'07 truck with coil front suspension, you will have better turning and off road capabilities.
If he's wanting to be serious about building a puller(which i assume he means sled puller) then stock parts don't mean ****. So that means either a built Torqshift that you hope will shift right and not have issues, or you can get a BTS built/PCS controlled 4r100 that will make 5r's cry all the way back to the trailer. Powerwise to hit over 500hp any engine will take work. 600hp isn't that
difficult with a 7.3. Your talking dual SD pumps, decent HPOP, 200% nozzle injectors, decent sized charger, studs/springs/pushrods, and if your lucky and have good tuning the stock forged rods should hold for a while. Not sure with a 6.oh but i'm sure at least injectors will be needed for 500hp, and a similar parts list to hit 600.
Suspension-wise a early 6.0 or 7.3 will be a better puller as well. The coil SD's hop way to much in the front end, whereas leafs seem to stay fairly calm. Now of course if you went 3/4 link in the front it shouldn't hop at all either unless there is an issue.
Really need to know more about your goals/plans/budget to give much input though.
97 F-250 ECLB 7.3/E4OD/4wd
, 216k miles: FFD 250/200%'s, stock turbo, built trans, e-fuel, Swamps IDM, shimmed 910's, Melling LPOP, welded cam gear/modified pick-up tube. Built and tuned by me
97 F-250 ECSB project truck:250/2's, Gen 3, S468, SD axles, studs/springs, ZF-6