man, you can pull off both. while i agree with most of the responders that the boards are a bit too long and baggy, i don't agree with the recs to go for a higher cut speedo. the mankini is just plain creepy. personally, i find square cuts or a regulation briefs completely pounceable.
and you do fill them out nicely.
and it depends on where you are. lake michigan or the redneck rivera, go with the boards. mexico, miami or the southern uk, go with the briefs. i lived in brighton for a year and there was certainly no shyness there.