Scorewatch is an Equifax-monitoring service, and so it can only provide access to your Equifax reports and FICO scores.
As to when you should use the second report, that's up to you. For instance, if you have been working on your credit profile, and there has been a big change --greatly reduced util, a negative falling off, etc., you might want to check your scores at that point. I'd hold off pulling the report/ score until a couple of weeks after you think everything has updated. Unlike Experian, Equifax doesn't update our reports as soon as it gets new info, and you don't want to waste a pull if everything isn't in yet.
Hope that helps! 
eta: but that suggestion about pulling after there's been a big change is just that: a suggestion. It's up to you when you decide to do so again.