Yes, I think you figured it out....
for 10Steem you can get 1000 SP for 30 days or 333 SP spread out over 90days. (not realistic...just an ex)

Actually the 90day delegation is about 10-15% stronger or better... so if you can afford it... go for the longer delegation. but balance with your time input... if you want to go Hard and Heavy for 30 days, better to invest 10 or 20 or 50 for much more SP but only 30 days. But if you are slow and steady, less SP, but longer time is a better deal.