Careful to understand the way some advert the head height! The higher water has to go , the less flow, so when some say a max head of six feet, they may mean that the flow is zero at that height. Just another thing to be aware of when shopping. The best info is a chart that tells what the pump does at various levels. Not to say that one rated for a max head of six would not get it done at five but it could mean that there would be such a low flow that your might grow gray before getting it done?
The pump natemcnutty referred to has good info, so it then depends how much time versus how much water you have to move. I tend to figure the info will be under "ideal" conditions so go for the next larger pump than I calculate needed.