Hardy waterlilies bloom more than tropical waterlilies in a cooler range of water, while tropical waterlilies will like 90's waters, hardies will go heat dormant and stop blooming.
It's fairly easy to think one blooms better than the other, when seeing them in different temperature range settings.
Oh, and hardies bloom months earlier in the year with quite often a two months head start...
Regards, andy
http://www.flickr.com/photos/21940871@N06/http://s93.photobucket.com/albums/l42/adavisus/ 
