Either one!

It isn’t possible to give you advice on this question without a lot more information. Without understanding your background, goals, schedule, training age, etc, etc, I simply don’t have enough information to adequately answer your question.

I am with Dan, we need more info to properly guide you.  That being said, the template that would work best for you will likely be the one you will be able to adhere to.  Don’t pick the 4 day/ wk if you’re busy and have a demanding job and a young family.  Pick the template that fits with your lifestyle and adjust from there.  Just because you start with a 3 day, doesn’t mean you can’t jump to a 4day when your schedule changes, or vice versa.  You just need to know how to properly plan things when the time comes to make those changes

If all things are held equal, start with the conventional split and then do the combo split for another round and go back and forth. Moving the lifts around will keep things challenging enough for your body that you’ll continue to make gains