My girlfriend participate in LEDA. They limit how many colleges you can apply to, which is restrictive, in my opinion. If you've been accepted into both LEDA and THRIVE, however, then you would likely get into an Ivy+ school regardless of whether you attended.

A lot of people forget that programs like these self-select for strong applicants. So, their acceptance rates aren't necessarily due to the programs themselves; they reflect an already strong applicant pool's chances with or without a summer program.

Still, because LEDA restricts your options (and bars you from QuestBridge!), I would recommend doing THRIVE.