We went last year and did the reserved X-mas seating, and I was really glad we did. The reserved seats are excellent, front and center with great views, and Sea World does a pretty good job of policing this area to keep out people who didn't buy the pass.
However, if it would be worth it for you I think comes down to how many X-mas shows you want to see. We wanted to see all the Christmas shows, and I'm not sure about this year, but last year that was impossible to do without the pass.
They had the shows planned out so that there was only one combination of show times that allowed you to fit them all in, and it was pretty much back to back. For instance, one show would end at say 7:45 and another would start at 8:00. Even if you jumped up and ran out as soon as the first show ended, you couldn't make it over to the next theater before the doors were closed. They close the doors before the show actually starts as soon as the theater is full. Sometimes, they close them as early as 30 minutes before show time. However, if you have the reserved seating pass, they will let you in after the doors have closed. So without the seating pass, we wouldn't have been able to fit in all the shows in one night. The pass was nice; it really allowed us to maximize our time. If we needed to take a bathroom break or grab a drink or a snack, we could without worrying the doors would be shut on us if we didn't get there an hour to 30 minutes early. As long as we made it before the show started, even 5 minutes before, we were fine.
On the other hand, if you are a large group, the dining may be the better deal. Even in the reserved section, you would still need to arrive pretty early to get seats all together. We were able to arrive last minute because there were only 2 of us. A bigger group would have had to split up to fill in the few spaces left.
Also last year, the pass gave you QQ access to the Polar Express ride. Not sure if that is included this year as well or not. If that is a priority, it might be another reason to get the pass.
To decide which is worth it, you need to ask yourself these questions:
1. How early are you ok with staking out your seats? 1 hour? Would you rather be doing something else up to the last minute?
2. How big is the group? Will you need to arrive early to shows anyway?
3. How many shows are you set on seeing?
4. How much money is the dining plan saving you? Is that amount worth possibly missing a show?
5. How many meals/ snacks will you eat? Will members of your party go through the line more than once during the same meal? For instance, a good deal is a big group with lots of teens, men, or big eaters. A not so good deal is a small group with light eaters who will probably just snack all day anyway.
For instance, I don't think the dining plan will ever be a good deal for us. We usually eat breakfast before coming, have a light lunch or dinner, and a snack. for the other meal. Even when we do eat two full meals at SW, it still comes in way under the plan price. But I also know people who eat a lot, and it is a great deal for them, plus I think SW has some of the best QS food.
As far as the actual shows, my favorites were the ice skating show, the seal / sea lion show, and the Shamu show. Of which, the last two listed are basically the regular show with X-mas theming, so you could always skip them and just see the regular shows earlier in the day. That would only leave you needing to stake out a spot for O'Wondrous Night and the skating show early.
We didn't have any issues with lines for any of the attractions except the Penguins and the Polar Express. I suggest doing the Penguins first thing in the morning, and the Polar Express in the evening while everyone is distracted with the X-mas shows. The line for both get long quick and stay that way most of the day. However, everything else seems to be hit or miss fluctuating between walk-on and 20 minute waits. This was my experience for every trip to SW since last X-mas as well, even during spring break. I think Blackfish has really cut the lines down.