I prefer to chose tires that I'll run at about 100psi or less, and look for tires of the corresponding width. If you want to be very finicky, you can use a slightly narrower tire on the front than rear, but I don't bother.
If you're on the fence between two tires, go wider. This will buy you more margin of error protecting your rims form road hazards.
BTW- don't agonize over this too much. Among decent tires the difference best to worst is so tiny compared to wind drag at speed that it won't matter anyway.
i wouldn't take too seriously what most people say about tire width and performance. especially since it is so easy to make a reasonable determination for yourself.
i imagine most triathletes have various timing and distance measuring equipment, so what i would do is map out a rather flat, wind neutral, low-traffic, minimal stopping route. maybe even a park setting. set a heart rate target and keep it for a specified time interval. when done. see how far you have gone. switch tires and do it again sometime under similar circumstances. maybe a few times, then average and compare.
i like this approach because i wouldn't have to take some internet strangers hearsay advice.
BTW, before someone beats me to it, this applies to me too.
Davidad, thanks for the link to the longer article. I can never find it when doing quick searches. I've always been a firm believer in wider tires (within reason) and am happy to see my belief supported by good test data.
FWIW- all the data in the world will never convince the higher pressure rolls easier crowd, si I just put it out there and folks can do whatever they will do anyway.