There's a difference between men's swimsuits and men's swimwear, because the latter, though it sounds like an inclusive phrase, rarely deals with swimsuits; swimsuits are a whole other breed meant for surfers, cold water, a hopefully sexy body, or any of the above. Swimwear is frankly a fancy way of say swimming trunks.
The dreaded word: Speedo. To this day, the Speedo is as fashionable as it was in the mid-twentieth century when it comes to men's swimwear. The elusive Speedo rarely looks good, men's swimwear or any form of fashion. Years back, men who were in shape could proudly strut about with the Speedo. These days, strutting men with a six-pack look too feminine with a Speedo on, at least from the Americans' perspective. There's no good way of wearing a Speedo, and yet fat hairy men sport in a crowd that, for some reason, never reports the crime to the fashion police. Yet, there is another trend that is not necessarily European, but, instead global, or at least Western. When going to the beach, the pool, or just looking to get wet in public on purpose, swim trunks now sport a look that resembles shorts. In fact, a somewhat common style of trunks even includes pockets and colors to parallel cargo pants. Some even have the same feel, acting like shorts that can simply survive a good soaking. So, men's swimwear is now multitasking-one minute you're skateboarding fashionably, the next you're cannonball-ing in a no-dive area of the neighbor's pool, again, fashionably.
Some states, such as Hawaii, Florida, but more notably California thanks to its street cred of hosting all the hotties from Hollywood, Beverly Hills, and Orange County, take pride in the sun that presents the opportunity to go swimming almost 365 days a year and allow men's swimwear to be as common as running shoes. How? By making sure that no one confuses supposed cargo shorts with swimwear. Loud colors thrust upon flower designs mark the stereotypical swim trunks. The irony is that men's swimwear is becoming more and more popular as being part of the "casual look." Wearing loud swimwear with a shirt and going to class is a norm in the warm, sunny states full of wet fun. Becoming a badge of honor, swimwear is the new aesthetics. Sadly, such a look doesn't work everywhere thanks to the title of "poser."
Other than state loyalty, the same rules are applied to men's swimwear as they would to any form of fashion-don't be too loud unless you have the personality to back it up. Khaki or mute, solid colors on shorts that apparently can be wet present either a teenage, skater look or a more serious, business-like look, ironically. On the other hand, the loud Hawaiian-like patterns, designs, and general colors on obvious men's swimwear are great for kids or jolly or eccentric adults, both of whom run around the pool in a game of Marco Polo.
All in all, it comes down to subjective preference, but because men's swimwear comes in either mute or loud colors, and rarely anywhere in between without resembling the 80's, be ready to act accordingly-business-like or more eccentric. Both are a great choice, but simply be prepared.