There really is no difference in practice between the HHonors "may" wording and the SPG "guaranteed if there is availability" wording. There are many reports in the SPG forum that what the hotel considers "available" and what a guest considers it to be may be two very different things. For example, in many cases with the SPG benefit, a suite will be available for booking for cash at the time of check-in yet be deemed "unavailable" for the "guaranteed if available" upgrade. This can happen if the room has been blocked for an upgrade for a guest that they know will be arriving late. Some (many?) hotels take the initiative to go through all reservations for the day, looking for those that have late arrivals noted, and "block" the available suites for upgrades for those guests. Thus, they are are unavailable for upgrade to eligible guests who arrive earlier. And if by chance someone books one of those suites for cash before the blocked-for guest arrives, then they don't get the upgrade either, as it's no longer available to them either at time of check-in.