We did both Cafe Mickey and Inventions on our last trip to DLP, and both have some fantastic features, both are worth the money however both are two completely different dining experiences and the price reflects this in my opinion. Now my personal opinion on this would be to do both if your going for more than 2 days because they are both fantastic character restaurants, but if you can only do one, go with Inventions. More expensive but worth it.
We did Cafe Mickey for breakfast the food was great, and the decor is brilliant, you can't not go wrong.
HOWEVER, in regards to Inventions we found this to be on another level of character dining. Even more so than anything we've experienced in WDW.
One other thing to remember is that Inventions overlooks Main Street USA, so if you time your reservation just right (Like we did luckily), they play the soundtrack to Dreams over the speakers in the restaurant and you have an amazing view of Disney Dreams whilst having you're meal.
The food was fantastic and lots more choice than Cafe Mickey.
In regards to the characters, really this is the best character meet opportunity you'll find in DLP. We met Mickey, Minnie, Goofy, Pluto, Pinocchio and Gepetto. Each character will give you a very personal experience, rather than just a quick stop and a photo. Mickey even sat on our table with Goofy for a good 5 minutes interacting and taking lots of photo's.
The character interaction didn't stop there, we even bumped into Mickey and Goofy going into the dressing room whilst we were wandering around the hotel, and they interacted with us in the corridor. We are two adults and we thought this was such a magical touch, so if you have kids with you this would make their holiday.
So although Inventions is a little more pricey, for me, the difference in price DOES effect your experience. So I'd go with Inventions.
However as I said earlier, do both