Barring discard attacks and trash for benefit or multi-Prince shenanigans, princing a Scheme is strictly better than princing a Smithy because you can always at least Scheme the Smithy and play it as your first action each turn, which is functionally equivalent to princing a Smithy, but you can also choose to play something else first (say, Fishing Village - Watchtower) or scheme another card this turn. Hence a princed Scheme gives you greater flexibility.
I disagree that it's strictly better, because you're essentially sacrificing the Scheme card. In other words, if I'm using Prince of Schemes to play Smithy as my first action each turn, that takes up the Prince, Scheme, and Smithy cards from my deck to do it; whereas if I'm using Prince of Smithy, I still have the Scheme milling around in my deck (which is now fast-cycling) to use additionally on something else.
That said, Prince of Scheme is awesome when you have nice $5+ cards.
Are you still working for Goko? If so, are you guys working on implementing this card soon?
Oh, my profile! No, no, I left Goko last year. But Goko isn't implementing anything for Dominion Online anymore. Making Fun took over hosting it. I've joined Making Fun, because I love Dominion. I'm the only one though: none of the other 30 or so people who were at Goko at one time are involved in Dominion Online now. It's an entirely new team. The code is holding us back is the code :-) but we'll have an announcement on that shortly.
Anyway, one of the things I did is write the AI, so yes, I'm just starting to figure out how Prince should play in order to implement it in the AI. Which cards to play is always interesting... here is a start:
Awful: Prince (duh!), Tactician (>$4), duration cards, Island, a knight (unless you got it from the Black Market deck instead of a knights pile), Feast/Deathcart/other self-trashing cards other than Hermit
Bad because really dangerous: Remake, Rats
Pretty bad because dangerous: Bishop & other forced trashing cards
Might be bad due to forced play of actions: Throne Room
Suboptimal: Shanty Town, Menagerie, Treasury(>$4)/Walled Village, Fortress, Watchtower/Library/JoaT (unless there's a discard attack)
Good: everything else -- Monument, Smithy, Militia, Scheme, etc.
Awesome: Prizes