Under the conditions you describe, I would say "neither". The FCC has to be able to make a reasonable prediction of when you will be on the air, before they go to the expense of sending agents to stake out your area and try to DF you. Unless you're extremely unlucky and happen to fire up when they are already in town to do routine monitoring, you're very unlikely to be caught doing it if it's limited to about once a month, whether on shortwave or FM. They won't spend days or weeks in your town waiting to catch one pirate. If you indeed limit your on-air time and the frequency of broadcasts as you describe, the likelihood of ever being caught is extremely slim.
Once you get started-- it can be difficult to limit yourself that way, and I know about that first-hand. That's where you can get into trouble.... but, standard operating procedure for the FCC shutting down a pirate for the first time, is just to TELL you to stop broadcasting, and issue a "Notice Of Unlicensed Operation", again, warning you to cease broadcasting. They generally don't issue fines to unlicensed broadcasters unless the warning is ignored.
There are a lot of variables to this, such as:
are you close (in driving time) to any of the FCC Field Offices?
For FM, are you in a large market where licensed stations are apt to report you?
Or are you out in the boondocks with no local radio stations?
One watt of FM isn't going to go very far, so it's less likely you'd be reported by anyone at that power level, than if you were using 20 watts or 100 watts...
On FM, the FCC usually can't monitor you directly, unless you happened to be in the same city as one of their field offices, or close enough to one of their monitoring stations-- they respond to complaints (albeit rather slowly), usually being made by licensed stations.
On shortwave, they can triangulate you via their nationwide monitoring station network and get an approximation of your location... Then they would need to be able to make a reasonable prediction of when you would be on the air again, so that they could have a DF vehicle in place when you go on the air to track you down. If they can't make that prediction, they are nowhere. Even on shortwave, they usually won't try to take any action on pirates until there's some kind of complaint made by someone-- as they already have a long list of pirate complaints-- mostly for FM pirates in New York, New Jersey and Florida, where they can't seem to make any dent in the action.
On shortwave, the FCC is more likely to be aware of your activity because it could be heard on their long-range monitoring network-- but they are generally more likely to ignore it because enforcement against the many local FM pirates takes priority.