If you're a guy, take into consideration nipple chafing. I would think the looser top would cause more chafing. I figure the tight sports bras I wear are the reason why I don't get it. So, if you go with the loose top, get some nip guards.
For exactly the reasons Whit mentioned. You'll be in it for 4-6 hours, and anything moving around can cause abrasion. Nip guards and bodyglide help, but for me, I'd like something non-loose please.
Done the bloody nips thing once, don't want to repeat it. Let's just say sweat really stings.
In triathlons, I wear Tyr tops that I like vey much. I also like the Hind T-shirt or mock turtleneck-type tops, depending on temps. They are tight, self-wicking, and I've never had a nipple chafing problem with them. Come to think of it, I've never had a nipple chafing problem when I wear a tight sports bra, either.