There are both pros and cons for this question. More often than not, union labor is more expensive because of all the benefits that some non union contractors do not offer. However, with the union you get the affordability of more man power if needed from their labor pool.
Both union and non union contractors will be more than capable of completing remodeling work. Union contractors are more expensive due to all the benefits that are offered. However, using union labor, you can be assured that if the job calls for it, they have the available man power to complete the biggest projects.