Cheap power aside, how much more electricity will the 2speed use?
Low speed moves 1/2 the water using 1/4 the electricity as on high speed/comparable single speed. So even doubling the run time on low to move the same water is a 50% electrical savings. A VS pump is slightly more efficient at all speeds, biggest advantage being it can run at even lower RPM for more electrical savings ... but you have to be sure you have enough flow for equipment and skimming to work.
Is the low speed plenty for your solar system?
I have accidentally left my solar on with the pump on low (before my automation was installed) and it seemed to work ok. BUT, solar heating is most efficient at high flow rates, so running it on low will not be as beneficial. You would certainly want to prime the panels on high speed to ensure all the air was removed. Personally, when I am running solar, I run on high speed. When solar is not needed I run on mostly low speed.
Is it true that 2speed pumps outlast variable speed?
I have no idea. 2-speed pumps have been around longer and there are no electronic components that could go bad being out in the weather ... but maybe the VS are fine.
How much does yours cost you per month and what's your electric cost per kW hour?
All fees and taxes included, I am at about $0.12/kW in the summer. I have no idea what the pump costs to run. Unless you get over about $0.20/kW or have large VS rebate, it may take 4+ years for the VS pump to save more in electricity to compensate for the higher initial cost. My 1HP 2-speed Superflo is less than $450. The larger VS pumps are around $900.
Also, who makes the best/most efficient 2speed?
There is no real best/most efficient I don't think. If you stick with the major brands they are all pretty comparable ... you will find people happy with any of them and your will find people that have problems with any of them.