I bet there is a higher chance to get good heroes of you give up 450 gems instead of 150. Else they wouldn't have that option there. Or is it just me?
There is no proven fact that the 450 gem option renders a higher chance at legendary heroes. Both the 150, and the 450 gem options give the same chance percentages. The 450 gem option just allows you to use more gems at a faster rate.
It just feels like that when I get blue slimes both the times I pick 150. When I chose the 450 I got 2 heroes (Blue, sure) and a purple slime. I might just have been "Lucky". Have you guys managed to get a legendary by doing a 150?
It really just depends on your luck. I spent over 2000 gems (all free) using both the 150 and 450 option and got absolutely jack squat. I then started another account, spent 600 gems and got a Succubus, Pain-Da, Angel, and Druid.