I cannot see what is the difference between those two big brothers.
Some personal data, like bank activities and hospital journals, are encrypted in their own systems but collected, decrypted in PRISM. I cannot see which big brother is more evil.
The 大SIX has not choice but 入侵过 伤过 your computer. But PRISM can just collect everything throught your internet provider(s), i.e., your windows operating system, backbone network, your social network provider, etc., without any notice to YOU!
It is not real that your data JUST stored in X-Keyscore for 3 days. All internet service providers store all data for at least 6 months by regulation.
Furthermore, "GFW" does NOT 入侵过 your computer but filters traffics between 大SIX and outside.