23 or 25?

I've run 23's on both road and tri bikes (not that I've ridden either much in the past 5 years). 

Mostly because that's what was commonly available.  I'm not an overly sophisticated cyclist.  I've run cheap Kenda's and now have Gatorskins/GP4000's on both, mostly because I prefer to not fix flats either in training or racing. 

I'd guess if you run the same pressure, you're into secondary effects due to width.  At the same pressure, the force of the contact patch is distributed over a *slightly* larger contact area.  As such, the sidewall flex which I would expect to be the primary rolling loss factor would be slightly lower.  Again, I expect this is a secondary effect if the tire pressure is the same.  25's would theoretically be a few grams heavier multiplied by the radius would have slightly higher rotating inertia, but given triathletes should be doing solo riding, not too much of a factor. 

I personally have 23's on my training wheels and the Force/Attach 22-front/24-back on my race wheels.  The 23's are the same reasons as Mike, when I got into the sport 8 years ago that is what was available.  The 22/24 had some time saving advantages 8-years ago for racing and I wanted to take the advantage.  My Continental Grandprix 4000 used to be the best all-around option.  I would train on them and race on them.  They never got flats.  I would get them at great prices on sales.  They would last for every which is why I haven't changed tires in a few years even though I put a few thousand miles on them.  Technology has changes over the past 8 years.  The Grand Prix 4000 are oaky now but they don't even get honorable mention in the running for the best tire anymore.  The 25's are the best tire now.  If you are shopping for something new get the 25's.  i will stick with my 23s for another 4,000 miles until they wear bald and then get the 25's.  

25s and the GP5000s are my go-to at this point on both bikes.  Tires are a rounding error in cost of triathlon / cycling and make a huge difference in speed, handling, and comfort.  When I moved to the 5000s when they came out I was like, "whoa" they first time I rode on them.