Pump "efficiency" is related more to the size of the pump and the speed than anything else. So go small and go with a two speed.

But as long as the wet end is in good condition, I probably would just replace the motor and you could replace it with a two speed to improve the efficiency.

However, you can find new pumps that are only about $100 more than a motor replacement but then you are dealing with plumbing changes.

If you decide on a pump change and as long as you don't have any high flow rate requirements, spa, waterfalls, etc., then a Superflo 2 speed like this one might be a good choice:

http://www.amazon.com/Pentair-340042...words=Superflo