All I can offer is that I go two-piece because I like to have "emergency access" that doesn't involve stripping all the way down.

A good, practical reason for two piece!

I'm 185 so not small. I found certain tri shorts to dig into my hip flexors in the aero position but not all. This year I switched to a one piece Kiwami Konami and I have a Desota Forza Tri suit. I also like the Desota tri short but not the low cut ones. The Forza have a small 4mm real pad, which I like.

For me its mostly a combination of a good pad, front zipper, and a small pocket. I wore the Kiwami in Kona and it worked very well. I just wore it without any speedsuit for the swim and rode and ran in it. No problems. I did stop to pee at a porto john. The one piece can slow you down but I had it zipped down and ready to go when I stopped. Most of the time with tri shorts, I just let it fly, especially in a half.

I'd vote trying one of the tri suits I recommended for a half. I don't think you could go wrong. If you can pee in your suit and have no problem squirting some water afterwards, then go with a tri-suit. If you feel the need to stop at a port o john or in the bushes, go two piece.

I have tried both. What I have found is that for a suit to be good for me (ie aero), that it has to be very tight, especially in the torso. On the other hand, I have never run well in one of these suits. As such, I am a 2 piece guy now. That way I can wear a super tight front full zip jersey and then unzip it nearly all the way and have no compression on the upper body when running.

I always felt hot running in the one piece, even when the weather was cool. Just one guy.

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I can't stand a waistband digging into me while I'm racing, and I don't like fiddling with keeping the top where it's supposed to be -- so I prefer one-piece, even at 70.3 distance. Full disclosure... I'm not a dude. ;)

I like a two piece because all of the one pieces I've tried dont fit right.

I'm 6'3, wide shoulders and 32inch waist. Medium is usually too tight in shoulders, large is usually too big in the waist.

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If you can try both and decide that's gonna be your best bet..

I have found 2 pieces to be fine in water and on bike but the shorts when saturated have dropped and offer reduced support down below and can lead to chaffing.. (30kms into the run typically). This is less of an issue in shorter events... and perhaps less of an ossie if you have slapped enough vaseline down there..

One pieces stay in place for the whole event.. and in general i prefer them.. i found it make no difference to ventilation in real terms.. pockets and storage are often greater limiters depending on the kit used ( I use Desoto Forza one piece and Riviera 2 piece.. )

Also if you have to take a shit in the 5 hrs or so a half takes then perhaps take a look at your diet ahead of events, and get better at emptying out pre race ;) .... you good to go after 3 shits is a good guide.. :)

best of luck finding what suits you.

You got pros and a cons for each:

One piece:
Pro: No waistband! This may not seem like such a big deal, but when you are running off the bike you can sometimes (not always) get almost hernia-like symptoms across the abdomen where the waistband is, especially if you haven't got a lot of fat. It sucks.
Pro: don't need a swim skin in a non-wetsuit race.
Con: no emergency port-a-potty access. Takes a while to disassemble and then (at least for me) I can't zip up on my own and have to ask a volunteer for help.

Two piece:
Pro: Emergency access!
Con: Waist band digs in.
Con: Need a swim skin in a non-wetsuit race - although I did swim in just a two piece once and didn't lose too much time. That was salt water.

I wear a one piece in important oly races like nats qualification. Two piece in 70.3. I don't care what I wear in sprints. That's just for fun.

I raced my first half in a two piece, and then went to a trisuit for the next.... few... This season I have moved onto a sleeved suit. (1 full, 2 halfs, one more full in 3 weeks)

Not looking back - one piece is so much more comfortable, and the sleeves on the new suit make a big difference for someone as slow as me.
Just roll up a leg for a quick stop, and if you need a longer stop - well, the comfort for the rest of the race is worth the 30seconds it is to peel down and squeeze back into the full suit afterwards. (just pray it isn't hugely messy in the portaloo - if so - run another 2km to the next aid station :D)