I've seen a strength test posted somewhere on this forum that suggested that the 29 spline was on par with a D44. I've also heard that they're good up to 35's. Some people like to talk bad about them but I suspect that has more to do with a "must-mod" mentality than their actual robustness. There were certain years of them that Chrysler screwed up something with the shaft/bearing and they develop the "8.25 clunk".
I have a 29 spline, welded, with 3.07's in my trail rig (I know, working on changing that ratio), and I've 5-grand clutch dumped it soooo many times with 31" tractor tires. Wouldn't dare do that with a D35. I know they're only 31's, but they hook HARD.
27 splines are marginally weaker. Have a friend with a 27 spline with a lockrite and 33's, and he has beat on it hard. 280K miles on it and it's still going strong.
If you get one with a D35, it's well worth the $100 to get an 8.25".