Well, I can at least give you my experience with both over the last 10 years. Started off with the 833 (23 spline)in the belvedere, at the time it was about 4100lbs and drove it across country. Also had about a total of 400 passes and 50k miles. The best time I had was 11.75. Traction was poor (was still trying to figure out the car and still am). I shifted about 6000 with the occasional missed shift. That was frustrating since it always seemed to happen at the wrong time. But still thought of it as a street car. Then decided to put in the TKO600. It was a great street highway trans. Because I was using a 3.55 at the time the .64 to 1 5th gear was only good when you were doing 65 & above, I wished I had gone to a higher ratio 5th. Also had several hundred passes with that trans, but for me shifting above 5800 was iffy (again, also always happened at the wrong time). The best time I had with that trans was 11.30 (there were other changes). I twisted 2 slip yokes, cracked the main case, cracked the tail shaft housing. Other stick racers kept telling me to get a real racing transmission, but no, I was going to make it work. Had it proshifted. Worked great for about 50 passes then grenaded 3rd gear. Fixed that went about 50 passes & grenaded 3rd gear. OK, get slipper clutch, went 50 passes and grenaded 3rd gear....Got Jerico.
Well my opinion is that the 833 iron case is stronger than the AL case of the TKO. One thing to remember with the 833 is that many of us are trying to abuse them like they are new when gears/countershaft, etc. are 40 years old. You can now get new gears (Passon & others) or have old gears magnafluxed and replace as required. The TKO was a great street/highway trans, and I think good as a occasional strip trans (depending on your goals). The 833 I think will take more abuse.
If racing is the primary goal, go Jerico.
Hope this helps
Greg