All these answers are missing the obvious.

You can purchase 8 upgrades for 35K miles anytime you want. So if you don't have an immediate need for either the miles or upgrades, you're better off getting the miles now. You keep your options that way. If at some point in the future, you find the upgrades better suit your needs, you can always use them to buy the upgrades then.

If you keep the upgrades, you can't exchange them for miles later.