Dura Ace vs. Ultegra 
The major differences are that Dura-Ace has more gear choices, chain ring choices - including triathlon specific chain rings and more crank lengths are available.

Weight: The weight difference is 221 g or 7.8 ounces with Dura-Ace being lighter. If you are a professional rider, the wider choice of gears, chain rings and crank lengths might make or break your season.

Performance: Some mechanics will tell you that Dura-Ace has to be adjusted more often due to the light weight construction and precise tuning. On the other hand, it might have a noticeably better feel for an elite or pro athlete.

Price: Ultegra is significantly less expensive than Dura-Ace. For most of us, the 99.9%, the weight difference won't matter. Don't eat that extra ham sandwich. There, you shaved some weight and saved a lot of money.

Ultegra vs. 105
The major differences between 105's and Ultegra are weight and the 105 is available in a 52-tooth large chain ring rather than a 53-tooth.
 

Weight: The weight difference is 230 g or 8 ounces with the Ultegra being lighter. The major weight difference is in the crankset. The other components are very close weight-wise.

Performance: According to most mechanics, the Ultegra will probably wear a little better and will probably be a little more durable. That's something to consider for triathletes as many are not 130 lb. road racers.

Price: As for price, there's a noticeable difference when you are purchasing aftermarket. As a complete bike, it probably won't be more than an extra $150 between a 105 and Ultegra tri bike. Go for the Ultegra, save a 1/2 lb. It will also increase your bike's resale value and make it easier to sell when you want that next cooler bike.

Summary

All three of these groups will perform solidly. All three are 10 speed. In fact, due to Shimano's trickle down technology, the 105 of today is better than the Dura Ace of 10 yrs ago. 

Why 105 and not Sora or Tiagra? A few reasons are Dura-Ace trickle down technology, construction (plastic steel plate), weight (heavy), durability (lack of compared to 105), 8 speed (not 10 speed).

You can't tell from a ride around the parking lot. 105 components will let you ride more efficiently once you learn to use the added gears. The weight savings, added power transfer from the 2-piece crankset and increased durability will make you happier in the long run.

- Beginner Triathlete