What bones for beef stock
To get that gelatin you're looking, you need joint bones and lots of them.  The back, neck, tail are great for that.  If you've ever made Ox tail soup you'll know what I'm talking about as you'll get a really thick coating on your tongue from the gelatin that is disolved from the tail's connective tissues.

The reason for avoiding marrow bones is because it can make for a cloudy stock if the marrow gets broken up and the fact that marrow bones don't generally have the highest amount of connective tissues.  Cloudy stocks also are caused by boiling of the stock instead of simmering.  

Typically when making stock bases, you want them to be as clear as possible so you can use them for just about anything.  A cloudy stock doesn't make a consomme look very nice.  However, if you are just going to use the stock in items which don't demand a high amount of clarity then don't worry too much.

When you go to your butcher ask for knuckle bones(joint bones) and chine bones(back bones) as your first choice.