The T-19 is stronger (being made out of cast iron and not aluminum like the ZF), but IMHO the difference is close enough that I wouldn't let that influence me. To me, the more gears a diesel has, the better. I swapped a T-19 for a ZF5 in my truck a year ago, and it was like night and day. OTOH, my truck is mostly a freeway runner, so I really wanted the overdrive gear.
So, if $$ allows, I would go with the ZF5 unless this is going to be an off-road rig only...if it's going to be used strictly off-road, then the T-19 may be the way to go just for brute strength's sake.
And, to add a shameless plug
my T-19 and some related parts are for sale, if you decide to go that route...and I should be going down to L.A. sometime in the next month or two...