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I would never do either…
Only if all of your running clothes get stolen the night before and you cannot borrow anything from a friend.
I’ve run one in a one-piece. It was comfortable, and I didn’t have any bathroom issues. But, from a fashion standpoint, not such a good idea . . . just take a look at my profile 
WOW… just… WOW…
You’re wearing a tee underneath!
After doing a few triathlons I now do my run races in jammers and usually with a white long sleeve tight fitting top. If it’s loose I chafe.
I wouldn’t do the one piece.
I say wear splits and a singlet. way more comfortable than spandex.
… Yeah
Speedo is lighter ;-).
I would do top and bottom.
Unless you wanna place well,but if you have to pee It’s easier to pull down your pants rather unzipping the one piece trisuit it takes time.
Nerd Alert
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I get scoffed at for wearing my cycling jersey during running only events, but I don’t care. I like form fitting and I love having pockets in the back, for gel, arms warmers, gloves etc… And my nipples don’t get all chaffed like when I wear something loose.
I do prefer loose shorts unless it is raining, then tri shorts.
I don’t care what people worry about as I run past them though.
Only if i see the Kenyans do it …
On second thought, nope.
Please wear the suit. Get lots of pics and post on ST please.
X2 on having pockets while running. I often run train wearing a bike jersey just for the pockets. They really need to make running shirts that have pockets. Maybe they do and I have just not seen them.
pssst … RaceReady shorts. Enough pockets for all of your gels, keys, money, credit card, etc … and they bounce around less during the run than if the same items were to be placed in the bike jersey pockets.
You want pockets? Take a look at these shorts. I bought two pair and used them for all my long runs and the NYC Marathon. I carried 5 gels (one in each pocket) and my valet tag for my luggage.
That is fine. But I prefer my pockets in my jersey. I don’t like anything from the waist down. But that is maybe just me. I don’t even have trishorts with pockets. Maybe it comes from years as a cyclits and always having things in my jersey pockets. Heck I don’t even keep things in my pants pockets.
I wouldn’t care what anybody else thinks.
I would wear mine. I love the fit of a tri-suit over two peice. My comfort comes from having ‘bib’ type support for the weiners and beans. Things are comfortably snug and the shorts dont ride down. I also like the compression fit on my core, it holds the gut in (I lost the weight but the skin is still kinda stretched). The bathroom issue is a myth. I struggled to get into my tri suit the first few times, it’s a tight fit for me, but now it easily comes off when needed. Just make sure the zipper is on the front.
I dont know if they make a bib-short specifically for running. Not that I would buy one as a tri-suit is more versatile.
Let the haters hate. I don’t have a running background so I don’t have a vested interest in the culture/tradition/etiquette. The majority of marathoners around here are overweight soccer moms, weekend warriors, walkers etc, so as long as I put my training in and am in the top third of the field then they can point and laugh all they like. I earned the right to look like a tri-dork.
I guess I should put a small disclaimer in here for your sake: I never have, nor have any intention of ever running a stand alone marathon. I am doing a 50 Mile Ultra in the spring and plan to wear my tri-suit, but it’s likely under other clothing because it’s just too cold that time of year to go tri-suit.