Hard to pick between the two, because Boba Fett is my favorite character, but I think the edge must go to Luke. There are several different Fetts from other manufacturers, and the Sideshow version, while a welcome addition, is simply another one.
The Hot Toys Luke is far more important to the hobby, as Hot Toys' attention to detail and overall quality simply can't be beat. However, their prices are increasing rapidly, and with the Star Wars license, they could very well start leaving all competition behind and begin a long-lasting strangehold on the industry. Unless the bottom falls out of this Luke, it will signify massive change that will affect the entire industry. This is STAR WARS, after all.
This, I believe, would/could result in one of three scenarios for Sideshow:
1. Abandon the production of Star Wars figures entirely.
2. Returning to being a 'budget' manufacturer. Less quality, less detailed, less expensive. This would be a shame, and a backwards step for the company, because it looks like they are really making great progress in catching up to Hot Toys' quality.
3. Competing 1:1 with Hot Toys and whoever else, matching quality step for step. The dangerous part of this option is that it would likely lead to very high prices across the board. I would prefer this option, but especially after seeing the upcoming batch of figures at SDCC, the bottom line is that this hobby is becoming more and more expensive daily.
I think that, one way or another, the Hot Toys Bespin Luke will probably end being the most important and influential 1:6 figure in the history of the hobby, and I don't think that is an overstatement or being overly dramatic.