#define is textual replacement, so it is as fast as it can get. Plus it guarantees constness. The downside is that it's not type-safe.
On the other hand, const variables may or may not be replaced inline in the code. You can cast away the constness, forcing it to be in memory (although it probably resides in read-only memory to begin with, but there's headaches either way). It is guaranteed to be type-safe though since it carries its own type with it.
I would personally recommend const to make your intent clear.
