It may have changed but the big difference between the 250 and 350 was 2 things:
1. 350 had a 4" riser block on the rear axle vs 2" on the 250
2. The weight rating on the door sticker
Now there are a million different setups of these trucks with all kinds of various spring packages, etc, but the real difference is one you have heard several here say: the ratings. That is a double-edged sword as it affects both what you can tow, as well as places you can/can't go (and often your taxes). You cross that 10k threshold, and it hurts some applications. We have a short section of freeway here where it is 9k max GVWR on it. Can't even drive my 250 on it as it is 9400 GVWR. Those are the main reasons for the different models. Otherwise they are identical.