Its a matter of personal taste I think. Both are good products in their own way, but they don't compete in the same space IMHO.
Krispy Kreme's signature product is light, fluffy and thoroughly excellent when just off the line and still hot. Once it cools down, to my mind it loses most of it charm. I would rate it a 8/10 when hot, a 5/10 when cool.
Tim Horton's Chocolate Dip (and all of their yeast raised --vs. their cake-- donuts) is a thick, chewy tour de force. When fresh it is dense but soft and has just the right moisture content. The chocolate topping is just a bit soft and just a bit gooey. Absolutely incredible as both a taste and texture sensation. I would rate it a 10/10 when fresh. After sitting for a day on the shelf, the chocolate topping has become brittle and the donut itself has dried out a bit. 6.5/10 once it gets to this stage.
Both products should optimally be enjoyed at the peak of their freshness. In my universe, Tim Horton's trumps Krispy Kreme by a healthy (or is that "unhealthy" -- we are talking about donuts here) margin.