Well, if it's a long time from now, I'll be aged and frail. Not a whole lot of good money will do me then, and by relying on the Billion dollar bonus, I'm going to get lazy and careless with it, not investing a whole lot into earning additional money, so it would be a very finite advantage. Whereas the immortality/immunity serum would prevent me from aging, but presumably help me get stronger and maybe even healthier compared to my state upon taking it (even living into an age of advanced medical technology, potentially improving organs and systems that were deteriorating or have failed). I'd have many more opportunities in my prolonged life to legitimately establish a sturdy financial foundation for generations of off-springs.
Invincibility all the way.
"I sail through a golden nexus. By tanks with armor that glisten. I watch and I play with creations, and what I'm not reading, I listen." <-