I'll be the first to say that spools/welded diffs are kick-ass for off-highway/trail rigs. If he does any significant amount of his driving on road, however, he will likely begin to hate the spool and wish he'd gotten a locker. Lockers are better IMO on street/offroad rigs because they allow the axleshafts to unload in the form of allowing one wheel to spin faster than the other around a corner. In a spooled/welded "differential" (an oxymoron of sorts), the rear end has no way to unload and your tires will skip, hop or scrub to take up the slack around most every corner taken. Lockers allow for a much, much tighter turning radius in parking lots, when doing u-turns etc., for the very same reason. They aren't too horribly difficult to get the hang of, either. All it takes is throttle control and a little restraint.
Mini spools are dirt cheap for the 9in (about $20 last time I checked on Randy's) but I think a locker would more than pay for itself in extended tire life, rear-end parts and just plain drivability in the long run. Just my .02