Personally I wouldn't bother with spinners for driving games. Get multiple steering wheels or don't.

The modular solution is very interesting. A bit fiddly for my taste but otherwise there's not a lot of compromise. If you want a particular layout for a particular game, the bottom line is you can do it with modular. It's great how Doc's panels still fit within the compact, closed design, too...
Personally, though, my philosophy is to not try to support
everything in one cabinet. The different controls are just too diverse. Every time I try to plan out two or three control panels that won't bust the budget but that will play a good selection of games there's one more on the horizon that I find myself wanting to support. Like my six-button fighter layout expanded to eight because I wanted Neo-Geo to have its row of four and I didn't want the two rows mismatched.

Smash TV is tempting as well (I may support that on my four-player panel), as are two-player Marble Madness and dual-spinner games - but I have to draw the line somewhere. If you go with swappable panels there's
always going to be something you can
almost support.
Personally, I'm going to stick to swappable panels for now. I like that each swappable control panel will be "whole" and seamless, and they're going to have panel art and plexi, and a few other special features... modularity makes that harder. I think the modular panels will look better. And I prefer to treat control panel changes as strictly a "special occasion" thing - not something I'll be doing all the time.
If you want more flexibility than that, I think modularity could be a great way to go. Otherwise you're still in the same boat as me: constantly trying to draw the line between supported and unsupported games.

Of course, you could always combine the two approaches, too: have one of your swappable control panels be modular, or have a modular panel with some full-panel modules.

Whichever way you go you can potentially get all the advantages of the other.