I looked into blocktrades too. I don't remember the exact numbers, but you get more SP for your STEEM at minnowbooster. It's because the lease terms are more flexible. You can adjust how much you want to spend and for how long, etc. It's true that the blocktrades option is easier, faster and you're guaranteed the lease. But since I didn't mind waiting and I really wanted to get more for my STEEM, I went with minnowbooster.