I already have a business, but am deciding whether to go out and look for funding? Or bring on an investor who can provide their own resources?
I think it trinkles down to how your biz is currently performing outsourcing for funds means you won't have problems running your business after getting the funds.However if you feel managerial roles are also weighing you down then I'd advise looking for an investor whose also bringing in experience.
In most company setups, I don't believe a co-founder and investor can be chosen as alternative routes like that - if a business truly needs funding, you'll likely need a founding team who know their chosen areas well in order to achieve that capital. If you can cope with a non-investor co-founder, you didn't really need investment to begin with, you just needed certain skills you didn't already have on-hand, and in those circumstances it's almost always better to go for co-founders in my opinion. For one thing, it's far harder to source capital.