You haven't said how you feel about Shadow so hard to say if you should sell to raise cash.
IMO, unless you dislike Shadow, keep it and buy TSPP. I think it's one of the all-time best Stern pins that can keep you busy for a long time. You like the theme so I think you will like exploring the many ways to play it and the depth of the ruleset. Top notch programming on both from KJ though.