Art,
My advice would be to go with the 40 now and forget the 20. You have the same low end gph, same footprint and all the upside of flow at your disposal. They are about $100 difference between them and last I heard the upgrade kit was $150. So if there is any chance of you wanting to upgrade, its more cost effective to buy the larger pump now and just not use the added flow until you need it.
You can also find great deals on the 40's and get them under $400 new if you look around enough. Used you can pick them up quite a bit cheaper.