What's the real difference between a deluxe portable toilet and a flushing unit?

A flushing unit and a deluxe unit are usually the same kind of animal: a single stall sitting over its own holding tank. Ask your rep to spell it out, because what you described as the flushing option sounds like a restroom trailer, and that is a different thing entirely.

Here's how it shakes out from the weddings we've hosted at our barn. A flushing unit uses a foot pump to rinse the bowl with fresh water, so guests never look down into the tank. That alone kills most of the "ew" factor. The deluxe usually adds a sink and mirror but the waste still sits in one tank under the seat.

A trailer has several stalls, real porcelain style toilets, lights, a shared waste tank, and steps up into it. It needs a level spot, truck access, and often power.

For 150 guests with drinking, the trailer is the one people actually comment on afterward, especially in the evening. If the budget says no, flushing units with a separate handwash station still beat basic units by a mile. Just make sure the count works for your guest list and hours either way.