I own both and I like Monopoly more than most people do, but I think Whirlwind is a better game overall. I wouldn't agree that it's that much more shallow than Monopoly, to be honest. Whirlwind is a pretty feature-packed System 11, and Monopoly is a somewhat shallow DMD, so they're actually kinda close in terms of depth of rules. The only thing Monopoly really has over it rules-wise is the modes, which generally consist of "hit any ramp" or "hit the bank over and over."
I like the art and music/sounds on both, I just like them more on Whirlwind. Whirlwind also has a little more interesting flow due to the dual upper flipper shots -- it's just satisfying to knock out combos like the inner loop -> ramp, or better yet shoot through the pops -> inner loop -> ramp. The equivalent second upper flipper shot in Monopoly is a scoop that simply holds the ball and then drops it in the pops, which often results in a drain when the ball comes shooting out of them. So it's both a flow breaker and frustrating when it screws you.
I also find the system of lighting lock through shooting compass directions in Whirlwind to be more fun and challenging than just hitting ramps repeatedly until passing "Go" in Monopoly. As for toys, the spinning discs in Whirlwind are somewhat annoying but not as much as the "blocks the most important shot every time you need it" Cop Drop in Monopoly, or the much maligned spinning flipper.
Overall I'd say Monopoly is a lot of fun, integrates a lot of cool parts of its theme and is definitely under-appreciated, but Whirlwind is a true classic.