If they're confident that the 45mm fenders can fit between your seat stays and fork, I'd probably go for the larger fenders, just in case you ever want to change tire widths. I also like to buy fenders in the store so I can at least visualize how they look. Some fenders accommodate multiple size tires (26", 700c, 29") and look pretty bad on any but the largest wheel size, though I'm sure they still get the job done.