Hey,
IMO when it comes to redirects/merges on a website, the first thing that you want to ask yourself, whether the content that you are merging is more related to your homepage or inner page?
What I am trying to point out is, if you have two websites: one which was selling independent products/product groups, the other the branded website, I would create a new inner page/select an existing inner page that already would host the new products on the branded website and redirect it towards that page.
If we are talking about two websites that were hosting almost the same products, then I would go for a more detailed redirect, such as independent product pages get redirected to their correct version on the selected domain. If this scenario cannot be applied, only then I would redirect whole domain to the homepage.
Like mentioned above, this is my opinion, but I hope it helps you.
Gr., Keszi
(if you already have a bing ads account)