Everyone will say the Wishes dessert party is better. We did Wishes in January with our boys (ages 15, 14 and 8) and we enjoyed it, but we're very likely to prefer the Illuminations party we have booked for the end of this month. If that's the case, we'll be in the minority for sure.
Seating: Regarding seating for Wishes, we were in the very front row and had an outstanding table. We knew that when the fireworks started, we'd need to stand to see because everyone gathers by the railing. That's absolutely fine with us. We didn't mind standing then and we won't mind standing for Illuminations. By design, we arrived about 20 minutes before Wishes started - that's about all the time we needed to eat a few desserts and watch the castle projection show. We sat briefly to eat desserts while watching the castle projection show, and then we stood with everyone else to watch fireworks. So for us, the difference between our Wishes experience and our Illuminations experience will be about 10 minutes of sitting. For those arriving earlier for Wishes, the difference between Wishes and Illuminations would be more significant.
Desserts: The Wishes party offers a serious ton of desserts - the amount and number of desserts is comical (at least to our two older boys.) The mass of crowds around the dessert tables was interesting - especially related to chocolate covered strawberries. We tried lots of desserts and it was a ton of fun. The desserts were all very OK. Our oldest (age 15) said he's rather have fewer desserts that are more interesting than the ton of desserts available at Wishes (some seemed very similar to the CL desserts at GF, Poly and CR.) We may find that at Illuminations - we'll see!
Viewing: Honestly, we're not really big into Wishes, but we love Illuminations. And we don't necessarily need a great viewing spot for either. For Wishes, we just wanted to avoid standing in the middle of a huge crowd - mission accomplished! For Illuminations, we know of a number of great viewing spots, and recently, we've just enjoyed walking around World Showcase during the show anyway. We're just doing this for the experience. And I don't mind the wine, although I wish it weren't sparkling. Can't have everything . . .
So I'd say the answer depends on what you enjoy. If you love Wishes, a spot to sit down before the fireworks while eating, tons of desserts and the lower price point, Wishes would be great. If you love Illuminations, don't mind standing, enjoy some sparkling wine and a few (but not a ton of) desserts, and don't mind the higher price point, Illuminations might be work well.