Wedding planner and decor rentals

Your vendors are doing the same thing as one is cheaper than the other. You can use each vendor for what you feel will make your day complete. The rental you like for the vendor, use them as that purpose and the planner for the other. There wouldn't be any conflicts within each others.

I think it's totally fine if you use both of them if one is cheaper for the decor

I agree, using one for décor and one for planning shouldn't be a problem. With my planner I'm personally working with she said she would help set up my on décor that I have purchased myself. So I think your safe to do what you want

I separate planning and decor as two categories so if you want to use one for planning and another for decor, that's totally fine.