So two things stood out to me in your original post. Gameplay and flow. Like others have said, GB is out of the running since it is not a flow game at all. So it's down to ST and GOT.
Both have incredible flow and great gameplay. I give the nod to GOT in both departments though. Here's why:
Flow: GOT pro will literally play as fast as you want it to. I have yet to have an airball fly off an orbit or ramp in close to 2,000 games. ST pro will play as fast as you want too, however, there is no incentive to do so that I am aware of. Yes, it has a combo feature, but nothing like the GOT combo multipliers. GOT wants you to play balls to the wall and nonstop shooting, and if you make your shots while playing this way, you will be rewarded handsomely.
Gameplay:
I am no expert on ST rules (get with SKB), but I have made it through almost every mode on that game and I find the overall gameplay to be less than GOT. GOT's rule set I know like the back of my hand, so I might be a little bias, but I have way more fun advancing through a game of GOT than ST. GOT just makes you think a hell of a lot more IMO.
All I do during ST is advance towards Kobiashi Maru, and when it's lit, loop the warp ramp until I have double scoring, then start it. I haven't seen many other rewarding ways to attack the game. I have seen that some people attack the modes by beating all three versions of the mode before moving to the next one. That could be cool, but I've never done it.
GOT's gameplay changes for me each time I pick a different house. I have developed a different attack plan for almost every house in the game so that makes it really fun. I also will take different paths to the same end (IT wizard mode), as they present themselves based on my play. Kind of hard to explain so I hope this info helps.
Either way, you will like ST or GOT, but my vote is for GOT Pro.