Someone will pay big money for either card, so you need to factor that in as well. The 5 looks great to me, personally, and I have no problems with off-centered cards that do not look horrific, so the 8MC is still quite delicious. It's a toss up.
Other than which one you like better, see what each card should/could/would bring you dollar-wise, and figure out what you should/could/would do with that money, and then make a decision on which scenario (selling the 5 or selling the 8MC) plays out the best for you at the moment.
If possible, I would also take your time. The Seaver RC is monumental and will always continue going up in value, so the longer you wait the better you'll do.
And if you want to trade, I have a mid-grade Gregg Jefferies RC I'd swap for one of them.
