Of the three, TWD is my favorite. However, I've only played the LE version, and with only the Pro model in my price-range (and being unsure if I'd like it) I've held off on going for one. Not what I'd call a flow game, but it's challenging and I like the toys. I think the modes feel unique.
ST Pro I own. I think it is a very good flow game, and the mode model offers a lot of scoring strategies to explore in terms of risk/reward (having 18 modes will do that). Modes definitely can feel repetitive. Decent ball times; easier game than TWD by far.
I've played an AC/DC several times (Luci version), but other than the sound/light show I never really got into it.
Anyway, I voted for ST in your poll given your indicated desire for flow and a desire for something less difficult. I have the longest ball times on ST, but with 18 modes I think it offers a lot of depth. Biggest negative will be the repetitiveness of those modes (I think it feels the most repetitive of the three).
Anyway, good luck deciding!