For beginners, we recommend the niche store as it contains multiple winning products and it will be easier to test multiple products in the same niche, in addition, you have the upsell advantage to get a higher average order value if you use upsell. And in case if at the first ads trial, a product doesn't get the expected you can test another product from your store without adding any other product in the store. One product store is still a very good option, with the niche store, you statistically have a higher chance of success.