If I buy a HP steel tank is there some reason why I should switch to DIN?
Not necessarily. You can put valves on steel tanks that afford you the ability to use a DIN regulator, or a yoke regulator with the use of a valve insert. And, although a general argument could possibly be made for the use of DIN over yoke in general, most training operations, and most rental outfits, in the Western Hemisphere use AL80s which are generally going have yoke valves.
Also, what size steel is similar in size to a 80AL? Outer dimension, obviously not volume.
I am not aware of a HP steel tank that is identicial in external dimensions (height and diameter) to an AL80. The HP 100 which several posters have mentioned comes very close - same diameter, but is ~2 inches shorter than an AL80. If what you want is a large single tank, you could also consider a HP130 - same length, but an 8 inch diameter instead of a 7.25. We have several DMs who use 130s for classes, for the very reason you indicate. They don't have to change tanks very often, and some go through 4 OW dives on one tank. Great tank for the right diver. Of course, the other 'dimension' you might take into consideration is the tank weight when full. The full AL80 might weigh somewhere abround 38 lbs, the full HP 100 might come in a bit over 40 lbs, while the full 130 will probably weigh something over 54 lbs. Actual weights may vary according to manufacturer.
If you are interested in comparative dimensions, the XS Scuba website offers some charts (only one manufacturer, though) as a guide:
XS Scuba Worthington Steel Cylinder Specifications
Who makes the best steel?
Who makes the best automobile? There are different opinions, based on trim characteristics, and whether the tank is painted or galvanized, etc., or primarily based on what the person owns. With the demise of Pressed Steel, which was a very popular line of galvanized HP steel scuba tanks, Worthingtons seem to be the most commonly seen in the US. That doesn't make them the 'best', just among the more popular.