.co.uk and .com redirecting

Hi I am a newbie and want to start my online shop and based in UK but I do want to sell worldwide too especially US. So I was going to buy the mydomain.co.uk and mydomain.com and use wordpress and woocomerce. My question is I know you can do something called a redirect 301 , by directing one site to the other , which way around do I do that.
As in do I keep the .co.uk as the main primary site and direct the .com to it. Or do I do it the other way around? And is this the best way to sell in both countries by having the .com and .co.uk and redirecting. Or am I supposed to have 2 separate sites?