I've never been to MVMCP but I did attend the Star Wars Dessert Party on May 11th. It was a "nice to do" once but not something we'd do again. The desserts were great - the atmosphere was fun - especially meeting BB-8 and Chewbacca but I thought the fireworks were ok. Perhaps it was because we were "too" close or that it was the last night of Wishes and I was thinking about that or maybe my expectations were just so high. Whatever it was, we didn't think it was worth the $276 we spent. But take all that with a grain of salt, it was only my opinion/experience. Have fun whatever you decide.
I vote for MVMCP! Its a much longer event and you can meet some special characters, see the parade twice, a few stage shows, Holiday Wishes, and have short waits for MK attractions! You also get free cookies and holiday beverages. Dessert parties provide prime viewing spots. If you camped out early for the Star Wars show (probably even when the Dessert party asks you to arrive), I bet you would be able to see it just fine.
We've done both. MVMCP is a lot of fun for the kids - great special fireworks, treats, parades. It's a big event, fun to run around at a time when the park can't be overrun with crowds too. I took my boyfriend to the Star Wars Dessert party and we really enjoyed it. The specialized desserts, the Chewbacca stein, the creativity with the food was amazing, and the fact that it's all you can eat and drink, even an open bar, feels pretty decadent. But you are herded to a special area, of course good viewpoint for the fireworks, but it's short and maybe not a thrill a minute thing for the kiddies. And it's totally focused on the fireworks - at the same time there was a dance party going on just across the way - which was loud and had characters and would have been fun to go over and enjoy but it felt like we should be standing around waiting in the roped off area for the fireworks -- a little weird.
Hope some of those details helped you... If I had to choose for myself, top would be the Villains Club (you didn't mention that but if you ever have the opportunity, that blows the other two away! Then the Star Wars Dessert Party and then MVMCP. But if it was for the kiddies, MVMCP, The Villains Club, then the Star Wars Dessert Party