Whatever one you currently have, switch. Especially if the other company is offering a discount for new subscribers.
When your discount is about to end. You need to look and see what the competitor's price is. If it's a deal then you can either switch, or you can call your current provider and threaten to switch unless they continue your discount rate.
If they don't believe you, then switch otherwise you get to keep your current service at the lower price.
Unfortunately you need to repeat this process yearly, or every time your discount rate expires.
My understanding is that if you show you are willing to switch, then not only will you tend to get lower prices, but you will also tend to get better service and connection speeds.
It is far cheaper to do this than to ever stick to a single vendor.