Assuming no spa, waterfall, or other water features: you want a 3/4 HP or smaller 1 HP two speed pump. 3/4 HP two speeds are rare, so usually you are forced to get a 1 HP. That will have the lowest lifetime cost and perform well with most pools, including yours.
Jason, I think you just helped answer the question I came out here for. I found out today that Mom's pump went kaput and I am looking into suggesting a 2-speed pump. Currently she has a 3/4 hp single speed Hayward 750 (that has had issues almost every year) for a 15k gallon gunite pool with a Hayward DE-3620 filter. I was thinking that a 2 speed 1 hp sounds about right for helping her save energy while getting circulation for longer periods of time. The pool is 75% shaded all day long by oak trees (shedding stuff) and I don't like the current routine of running 1 12-hour cycle each night (8pm to 8am). Just trying to figure out the Hayward offerings alone is mind boggling. I don't like the stress of making this type of decision under a time pressure.
BTW, I have always been kind of intrigued at the success you've had with 1/2 hp for 19k gallons. That seems low compared to most suggestions I've noticed.