If it's a case of a guaranteed seat in business in J on the 777 or a possible/maybe on the A380, I'd go for the guaranteed option everytime. Don't forget that you'll also get the chauffeur pick-up/drop off if you book more than 48 hours in advance too - something that you will miss out on if you go for a last minute upgrade. . .
Just my 2p though.
Regards,
JGX