Myself, I would rarher have the OBC. What they call an upgrade is not one to me..
With Early Saver I do both now. I get the "updecks" and mid-ship location I want, then get the OBC's when that category price is lowered. Example; I booked Riviera Deck, I'm now on mid-ship Empress Deck with OBC's to boot! You have to really keep a watch on the pricing though.