The truth is that neither are very specific for dachshunds, or small breeds. They are both good in that they provide maintenance nutrition - which means that they are not formulated to be high in calories.
The most important thing you want from a food is that it is low in calories and doesn't cause Gizmo to get obese. Lower back problems and other orthopedic problems are a concern with older dachshunds. Keeping them lean is the best way to avoid orthopedic problems in the future.
I think it's fine to keep feeding him the Canidae if he likes it and tolerates it well. You will want to consider switching to a senior food when he reaches seven years of age. These diets are formulated with even less calories to match the decrease in activity that often happens around that age.
For more information about finding the best food for Gizmo, I recommend you read the information at the link below.
https://www.petcoach.co/article/what-to-look-for-when-choosing-your-pet-s-food/I hope that helps and all is well with Gizmo.