You should not vent out the soffit if you have soffit vents. The moist air will be sucked into the soffit vents, and cause just as much (or more) trouble as venting directly into the attic.
The best option is to vent out the gable end of the attic. Make sure the ducting is sloped slightly back towards the fan, so condensate can drain. Use rigid metal ducting, not that flexible plastic junk.
IMO, and IME, bathroom fans should always be vented outside. Bathrooms are one of the biggest (if not the biggest) producer or water vapor in your home. When water vapor is trapped is can cause things like mold, mildew, damage to furniture, added difficulty in conditioning the air, and many more.
Just because a bathroom vent is not currently vented to the outside, does not mean it can't ever be. The house I'm currently living in had no ceiling exhaust fan or light (just a vanity light).
In this situation I was able to wire in and install a new unit, with manufacturer recommended vent line and exhaust this line through the closest soffit.
Depending on the layout and positioning of your bathroom, you could run through above joist spaces until you reach a soffit, run into the attic and then out a vent, or simply through the nearest exterior wall. Many times the shortest route is the simplest.
If this is something you aren't comfortable with, I highly suggest finding someone who can like a general contractor, trusted handiman, etc.
Keeping excess moisture out of your home will pay large dividends in the problems you're preventing.
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Only caution would be if it's actually in the floor, in which case you might want to consider precautions from liquid water going into it. Down low it won't vent moisture as well, but since that's not a concern for you it should not matter much - right behind the toilet bowl would appear to be your ideal intake position. Negative pressure is fine in ducts unless you are using a jet engine rather than an inline vent fan.