I would go with the spread personally unless you have a better/more clear thought as to how far and how fast/slow the bearish move will take place.
If you anticipate a downward draft to the underlying (downward moves usually happen faster than upward as a rule of thumb) and can watch/baby sit the position, a simple put would be good, maybe best. If you just anticipate a lingering downward move over the next month, not so much event/news driven, then the spread usually will work a little bit better.
To me it sounds like you have a good feeling as to the direction and amount, but not so much the time period.
Of course this is just a from the hip response since there are many more nuances that could be discussed, but I'm sure there will be others willing to address in more detail as well as specifics.
Good Luck,
and as always, just my .02