For a situation like this, code readability and adaptability is much more important than performance. Even if one of these methods is slightly faster than the other, the compute time saved is not worth the extra time you will spend trying to make changes when the requirements for your project change.
If you have hundreds of variables, you also might want to think about completely restructuring your program. Use functions so that each variable is only defined and used where it needs to be, or use a different data structure that better matches your data.
Once the program is done, and if performance is still an issue, you can use the profiler to identify the bottlenecks. I guarantee it won't be the structures.
