Neither of these mills is very rigid. According to a former machinist I know, neither is a full sized BP. He called BPs 'hobby' or 'prototype' mills.
If you want the most rigid machine you can get in a hobby machine, think 940 bed mill. I had the CO version and for it's size it was an animal. But I digress...
The differences you should be looking at between the 835 and the 833 is the knee and articulating head on the 835. If you need/want this flexibility, look at the 835. If you don't need/want this, go for the 833.
I am moving up from a round column mill drill. I'm betting they are both a huge leap forward on rigidity for me.
The 833tv has a maximum height of around 20" spindle to the table just like most of the knee mills. That means the knee isn't a huge advantage at all.
The 833tv doesn't have the nod adjustment so I'm hoping it will be as easy to shim as my round column mill has been. The addition of the cant adjustment will make things easier.
I did see a video of a guy complaining about the rigidity of the 833t but he had the mill set up at near max height on a crazy setup.