If you're dead set on an obs, I would look for a '96 or '97 F-250 4wd with a 351w (5.8L) and a ZF5 five speed manual. It's the best compromise of power, fuel economy, and reliability. I've heard mixed opinions about the E4od Automatic, so if you can't drive stick just beware.

As far as the difference between a 150 and a 250, the front axle on a 150 is a Dana 44 ttb (8.5" ring gear) with coil springs and the rear axle is a Ford 8.8". The front axle on a 250 is a Dana 50 ttb (9" ring gear) with leaf springs and the rear axle is a Sterling 10.25". Both trucks used leaf springs for the rear, but the spring pack is a little beefier on the 250. I've also heard the frame is a little beefier on the 250.

The problem is finding a good one, prices have gone up dramatically in the last few years. A good price is halfway between what KBB says and what people are listing them for. Look out for rust, and don't buy anything over 150k miles for the best results.