I did both Tahoe last year and Boulder this year. My girlfriend and I arrived 8 days before both races.
Pre-Race: Being from Kansas City, both locations are a breath of fresh air. If you love the mountains then you will have a good time at both places. I will say that we had a better time at Boulder because of the options on restaurants and activities. Spent some time in Estes park, chautauqua park, hiked 3 or 4 times, and had many great restaurant options, my girlfriend and myself both being very picky eaters. The scenery of Tahoe has a more overall beauty to it in my opinion, and certainly has that mountain town ski resort feel to it. Had a blast in both places but Boulder wins the pre-race contest.
Swim: In a reoccurring theme, Tahoe blows boulder out of the water on beauty. Swimming in clear waters of Tahoe vs. murky boulder reservoir is no contest. However, Tahoe is slightly more challenging. At around 6300ft vs 5300ft in boulder, you feel that elevation difference. I had to slow myself multiple times at Tahoe and didn’t have any problems at Boulder. Even though it is harder, Tahoe wins by a large margin for the swim.
Bike: Again, Tahoe beats boulder by way of beauty, by a huge amount. The Boulder bike may as well be the farmlands of Kansas but at 5500 ft. The bike in Boulder was easy, really easy actually and really fast. That said, the conditions this year were as close to perfect as you can get for a bike ride. Tahoe was significantly harder, yet significantly more enjoyable. Long sustained climbs. I had a 33min T1 (Flat tire & Bottleneck in tents), an additional flat on course, nutrition plan went to shit, I was under dressed and freezing, yet enjoyed the Tahoe bike massively more.
Run: This one is close between the two locales. Tahoe wins on scenery again, but not by as wide of a margin as the other two disciplines. Tahoe is harder again, but not significantly. Cons for Tahoe: 6500ft, not as good of crowd support (still great, don’t get me wrong), finish in small squaw valley (not as many good eats and shops.) The crowds around Boulder creek path were massive, support every step of the way. I will say its a draw for the run, pick your poison.
Overall race: If you want a harder race but unbeatable race scenery Tahoe, hands down. If you want incredible race week scenery and better food, but an easier and in my opinion, largely less enjoyable race day, Boulder it is.
Side note: If you plan on Tahoe, make sure to have a very versatile nutrition plan, and a wide range of gear available. You need to change a lot of things on a 50 degree race day when you have been training in 90 degree heat all summer. Also, one week of elevation will only get you a minor amount of acclimation at both locations. Don’t forget to ramp it down on race day.