As to the original question, it really depends on what you are doing with it. Are you talking about front or rear 60s? If you've seen front 60s in junk yards send me a PM with what junk yards you go to.
Rear 60s are plentiful and cheap. If you're talking about front 60s the first thing to consider is what side the pinion is on. If you have a passenger side pinion I can't help much, I've never dealt with those. They are all low pinion (or custom). Driver side pinion 60s can be high or low pinion, with varying spring-pad width, suspension types, knuckle-joint types, and wheel bearing types. I think the majority of people would pretty much agree that high pinion is better than low pinion, kingpins are better than ball joints, and serviceable wheel bearings are better than unit-bearings. All these features are common to 78-91 Ford F350 SRW front 60s. Some are more desirable than others for specific applications. For example, I think 78 and 79 F350 front 60s have the same spring pad width as a YJ, which makes spring-overs exceedingly easy.
More information than you could possibly need about front 60s can be found here:
http://www.maxxis.com/products/automotive/product_detail.asp?id=5477