Most areas are locked to one power provider - the infrastructure cost gets prohibitive FAST for power providers to try to compete in the same area.
All the places I have seen that have multiple providers for a utility service use the same infrastructure. More or less, you'll be using the same grid, but you have some choice in who acts as the layer in front of the wholesale cost - which actually varies a lot over the day. Traditionally, it gets averaged out and you get a flat rate, but some providers offer reduced rates during off peak. Actually do the calculations if that's available since it often comes out cheaper for a 24/7 constant load like mining.