Genuinely difficult, this. Both stories had perfectly OK premises and one or two decent characters. Dead Men Walking told its tale competently, and Boo Cook is never boring to look at, but as people have said it never quite nudged the needle above 'memorable', even when the zombies broke loose.
Stalag 666 on the other hand was incompetently told, by both writer and artist, who conspired to fill the pages with ordinary-looking humans walking and talking. BUT every now and then you got a smash-hit panel of an evil lizard creature whipping a person in half with a future weapon, and glimpses of the greatness locked inside Jon Davis-Hunt that would emerge in future efforts from him. So it's far more memorable both because it was much worse, and, in odd moments, much better.
If the question is either, 'which thrill would I rather see again', it's Dead Men Walking, 'cos lead character Jude has potential to tell more tales. If it's 'which thrill inherently had the more interesting story', I'd probably go with Stalag 666.
Guess on balance, then, it's Dead Men Walking for me.
Now I'm off to ponder how it compares to that other much-maligned 'person dropped onto a prison planet' story, Universal Soldier...