I now own a DI and have ~30-40 games on a Wonka in the last several months.
Both seem like good, worthy games.
DI has a "simpler" layout that I think is more friendly to newer players, and it has the "moving electric guy" which anybody who plays can notice and interact with. The quantum theater is neat, but I think how cool it operates would go over most newer players heads. Even when I played it on location I was a bit confused by how the virtual drop targets, spinners, captive ball etc. were working, especially since they stack together and with other features like the drone crate drop.
DI seems like the clear winner in terms of Modes, they are easy to start, and reasonably challenging and clear with what needs to be done to finish them. I can't tell you how to finish the modes in Charlie; and i usually start them "accidentally" while trying to do something else. To be honest I need to go back and re-read the rules for Charlie.
In Wonka it is easier to start multiple MB's and ride them for days. That may or may not be a plus depending on your perspective but it is not uncommon to start 3 MB's in Charlie at once. Often when I do this my score will go up by a factor of 10; how important this seems if you are playing for score is hard to understate.
With DI, you at most can bring in 2 MB's I think, and compared to the mode points the MB's seem a lot more tame in terms of blowing up your score. Basically you can do well with modes, and you can do well with MB's. They both work together. I don't feel that way with Wonka so far.
I give Wonka the edge in terms of layouts that interest me as a pinhead; but i know that someone like my wife would find the shots a bit challenging, and my younger daughter could not see the shots from the upper right flipper, and barely see the upper left flipper.
DI wins on sound for me by a long shot; David Theil is "the bomb". Wonka's sounds are okay, but DI's music and sound effects package are top notch. Some don't care for the DI callouts, but they fit the theme and to me are clearly better than Wonka's announcer.
DI also wins the display animations by a mile; i don't think a game comes close to the amount of effort and effect DI's art and animation package has in terms of creating the city; the displays on the phone, the quantum theater and the main display all work really well together and are all ambitious in their own right.
FWIW Wonka seems to be longer playing to me due to the "endless" MB stacking. That being said a lot of my DI in drains are from a really mean left sling into the right outlane. Even owning DI I get a lot of short balls, but some balls I can really get into a groove. DI is much, much shorter playing than the game I sold to fund it (LOTR); you might say "No crap" but FWIW it seems like a mid-length player, Wonka is more of a longer playing game for me.
I enjoy Wonka, I need to dive into some of the rules a bit more myself; they are different enough that either would be a good choice. I have said before that DI is the 90's game that I never knew I wanted to be made. If somehow this game was made in the 90's, with a production run of ~1500, it would be a $20,000 game no question. Wonka is the modern Lawlor game we all hoped he had in him. No wrong choices here.
I have officially written too much; if you read all of this, congrats!