There are no hard-and-fast CPU limits on DreamHost, but I doubt there are significant practical differences between amount of CPU utilization you can have at BlueHost compared with DreamHost. If your site impacts your shared webhost, it will have this impact wherever it is. What are you hosting, how much traffic are you getting, and how much CPU is it using, peak and average?
The only differences between those two are whether you want to move your domain registration to DreamHost or to leave your domain registration at BlueHost. Will BlueHost let you leave your registration with them when you don't have a hosting account with them?
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