Alright so there are multiple comparisons we can make here, but both can be pretty intense "sports". Triathlon is a little more of a type A personality sport and attracts a little more mature crowd of folks. Many people who compete in Triathlons may refer to themselves as "Triathletes". They say this as if it helps define who they are and what their purpose is. For example someone may say "I'm a father, husband, engineer, catholic, and a Triathlete" That doesn't happen so often with bodybuilders. I think they prefer to let their bodies do the talking. Sometimes this can be assisted by puffing your chest out or acting like you can't get your arms all the way down to your sides because your lats are too big.
Who is the most vain?
I gotta give this one to the bodybuilders, they basically only can flex their muscles if they have a mirror or if they are lucky enough, a crowd of people. All exercises are conducted looking in a mirror. However, it is extremely bold to just start hitting poses in a crowded gym, although, many folks really want to. Typically they wait until they get home from the gym, but they have to do it before they lose "the pump". If you don't know what the "pump" is then this should help. http://www.youtube.com/watch?v=iMjG2s6UOaw
Triathletes like to look good at races, but when they are working out they typically aren't thinking about how they look or how they will look. It is all about going fast, and they will do almost anything to go faster. If someone proved that rubbing mayonnaise on your feet made you faster, I guarantee you would see jars of it at races. Many racers see appearance as a competitive advantage, if you look fast, you will psych out your opponents and they will not challenge you.
Who is more dedicated?
Ok so this is a really close one, but I have to give it to the Triathletes. It is not uncommon to eat, breath, and sleep this sport. People get divorces due to triathlon training. Now THAT is dedication! Well, it is also totally screwed up, but I'm not comparing mental wellness here.
It is pretty rare to see a bodybuilder train more than 10 hours per week, but there are times during the triathlon training season that it is necessary to push much beyond that level.
Who watches their diet closer?
I gotta give this one to the bodybuilders. Any serious lifter knows (to the gram) what their carb, protein, and fat intake is on every day of every week. They are willing to get up in the middle of the night to eat cold chicken breasts from the fridge. They can recite the different types of proteins and when you should eat each during the day.
Triathletes are serious too, but just not at the same level. The focus typically revolves around "fueling" during races or workouts, or "recovery" immediately thereafter. Hydration is a concern, but tends to be more of a habit.
Which is more supportive?
This one goes to the triathletes. There are groups, forums, communities, fans, etc. that revolve around the sport. During races athletes encourage each other. After races they congratulate each other. There seems to be a trend for folks to point out their own weakness if it is your strength. The do that just to make you feel good about yourself.
In lifting, the support consists of spotting someone during a lift if they need it.
This is hard but it goes to the triathletes. The reason is a little more complex than the other comparisons in this post.
I find that many people who are competitive in endurance sports like triathlons do it because of the pain. They have something in their personal history that really hurts, and somehow the pain of a workout makes it feel better. On top of that you have the blisters, muscle aches, inflamed joints, black toenails, etc. that go with the sport. The pain of a workout lasts for hours.
Sure bodybuilding makes you sore and the workouts are intense, but it isn't as bad as triathlon.
Which is more dangerous?
Ok, so neither is extremely dangerous, but again this one goes to triathlon. This is mainly due to the fact that you could be hit by a car any second and get killed.
Bodybuilders, don't seem to have that fatal aspect to their sport. Probably the typical situation involves muscle pulls or dropping the weights on yourself somehow.
Which is more ridiculous to the "normal" person?
I'm not a very good judge of this, but I'm gonna go with triathlons being more ridiculous. Normal people regard triathlon workouts as "crazy" and ask why in the world would you do that? I just seems too odd to them to do a 3 hour bike-run brick workout. Although, running, biking, and swimming on their own are seen as quite normal activities. It is the desire to cram them all together and up the mileage to crazy levels, that's not normal.
Lifting weights is seen as a relatively standard way to get in shape. Bodybuilders just take that one thing to the extreme.
Which is more expensive?
Triathlon is clearly the winner here. Bodybuilding has the expense of food, supplements, gym membership, clothes, and training gear. None of that stuff is too bad on it's own or even added up.
Triathletes can and will spend 10's of thousands of dollars in a year to compete in their sport. For example, just a set of race wheels for a bike could be $2000. Sure, you could train and compete on a very small budget, but it seems pretty rare to have that happen.
Who shaves more?