I have only done a pool swim to date, so my vote is pools! The temps are great, no wetsuit needed. At my sprint tri last year, we had 2 people to a lane, so it was a bit choppy, but no worse than my usual swim. I train in a pool, and my swim rhythm is in 25 yards at a time, so I never thought about momentum before that wasn't 25 yards at a time. I think it can be an advantage to push off every 25 yards. With a good push off I can get about a quarter of the way down the lane. It is, however, a pain trying to get out of the pool. I swam past the line to get out (nearly crashing into the oncoming swimmers) to jump out after the steps. (check with race officials to make sure you are allowed to do before you do it ) Other racers informed me they spent almost a minute of my swim time waiting to get out of the pool. I want to sign up for a longer sprint tri this summer that will be an open water swim. I'll be able to better compare after that experience

edited for typo!

OWS, Baby!!! Pools wreak of chlorine. Lakes, rivers, and oceans wreak of The Great Outdoors. For my money, no comparison.

Also, I get sick to death of having to pass/get passed in a pool, pull up on the side to see how difficult it'll be to switch back and keep going, etc etc... I guess I'd rather get swum over in a lake, than in a pool. And you can forget about a group start in a pool. Group starts are the way to race! You can gauge where you are in the pack from a group start.

Mostly, though, I just can't stand wreaking of chlorine while racing.

Glad to see that open water is the consensus, because I am planning my first race, a mini tri, in October which includes an ocean swim. So, I'm assuming I should train in the ocean rather than the pool as much as possible?

I used to be a competitive swimmer so I did a lot of pool meets, but I cannot see doing them for a triathlon-and not because there aren't advantages-indeed, swimming in pools is easier insofar as you can go in a straight line because visibility is far better, you can gauge your distance better, getting kicked in the face will likely not happen as much, etc. But! there is just something about open water swims-I like the moderate brutality of it-which probably sounds crazy, but you swim on top and around people, when you complete it, you feel like you have done something remarkable...I just love it (though the wetsuits strangle me)!

I haven't done any events with swimming involved as of yet but I far prefer swimming in the pool to swimming in open water. I love turning at the wall, I love being able to see clearly and I freakishly enjoy the smell of chlorine (I know its just wrong and everything but I just do). I also quite enjoy swimming in open water but not as much as the pool, in both lake and ocean I find the visibility hampers me, its hard to plot a course and swim in a moderately straight line and if there is a current then it is extra tough. I also don't like the occasional bit of salt water in my mouth in the ocean and swimming for a seemingly endless amount of time without a fun wall to turn and spring off bores me. the brutality of perhaps getting a kicked in the face turns me right off, give me a more dignified pool swim any day.

I will never do another pool swim. OWS has a physicality to it, you have to sight, people can't misjudge where they should be seeded, etc.

My first try was an OWS and I hated it (mostly for lack of training) now I love OWS.
If you have never done an OWS before - be sure to at least get out in open water before the race. I dont care how far you can swim in a pool - open water is a different animal. Nothing like being mobbed by 150 of your closest friends

That's the best part man. The physicality of an OWS. Gets the juices flowing. Of course I post I'll never do another pool swim and I win free entry into a race with...... a pool swim. Typical.