There's no right or wrong answer here.
If it's cheaper for your corps, it'll be because your country is soaking up the extra costs.
The other thing to keep in mind is if your country and corp are generally ordering supplies at different levels of quality.
My corps usually order supplies at a quality of about 215.
My countries usually order supplies at a quality of about 120.
If you buy supplies with your nation, and then sell those supplies to a corp, depending on how many products you're selling and how central those supplies are to the final product, it could cause the quality of the final product to decrease.