I'd sure like to see some actual measurement data documenting the strength difference between these two type joints. I certainly don't doubt that the solids are stronger, but like Turtle commented, is it significant? And, what is the actual torque rating of each type? Unless you're dumping the clutch at high RPM, I would speculate that most engine/trans combos won't put out sufficient torque to twist off these joints. Up until a couple weeks ago when I got new halfshafts (and new Spicer solid joints), I have had greasable u-joints in my halfshafts for years. While I don't drag race, I do run the crap out of the car on autocross and road courses, all behind a big block and Hoosier tires. I've broken the transmission twice, but never had a u-joint issue. I realize YMMV.
I'm not trying to disagree just for disagreement's sake, but I just like to see actual data over bench racing speculation.