"Salvation" means "being saved from something" - with an obvious reference to something bad in some way. The students mentioned anything that is unacceptable, uncleanliness, sadness, pain, suffering, loneliness, etc. They defined "happiness" as joy, contentment, peace, satisfaction, lack of misery, etc.
We talked about how "The Plan of Salvation" is focused mostly on one's condition prior to being blessed - taking someone "from" one condition to another condition. The emphasis is on the condition "from which" the person is saved. It is on the "savior" who does the saving.
We talked about how "The Plan of Happiness" is focused mostly on one's promised condition of blessedness - taking someone "to" one condition from another condition. The emphasis is on the condition "to which" the person is taken. It is on the "blessed" who is being rewarded.
I then asked each of them to think about the two phrasings and which one resonates the most for them personally - and why. I told them that I wasn't looking for any particular, unanimous answer; rather, I really was interested in their personal responses. Five of the eight liked "Plan of Salvation" better, while the other three liked "Plan of Happiness". I used that to start a fuller discussion about why it's important to understand how things can be phrased differently to help people understand the same overall concept, since the overall message of both phrasings really is the same.
We talked about missionaries teaching investigators (or even inactive members) about Heavenly Father's Plan. I asked them how they could know which phrase we had discussed would be the most powerful for each person. One of the students immediately answered "the Spirit". I agreed with that but added that the missionaries could use both terms and ask the other people directly which was the most meaningful for them and why. With that understanding, other teachings could be tailored to the natural "orientation" of each investigator / member. All possible legitimate phrasings could be used, but understanding multiple phrasings could help increase understanding among differing people - and it's much more important to teach people than to teach specific wordings. ("Preach My Gospel" emphasizes that, explicitly.)
We then talked about people who sometimes struggle with either wording. The students came up with some really good examples, then I mentioned two broad categories.
First, people who are already happy, even generally, might not feel the type of uncleanliness, sadness, pain, suffering, loneliness, etc. that would help them feel a strong need to "be saved". For them, continued or enhanced happiness might resonate more deeply.
Second, people who struggle with depression, bi-polar disorder or any other condition that makes them feel like happiness is not possible in this life might really like a promise of future happiness, while others might feel much more strongly about being saved from their current condition.
Finally, we talked about how people who can't envision happiness in this life might feel like they are being beaten with a hammer if they hear happiness in this life preached regularly as the goal. Thus, even if they might like "The Plan of Happiness" that is focused on the next stage in life, that same phrase might be discouraging or even painful if it is focused on this life. Again, the end result is the same, but the unique focus of each phrasing might mean more to each person in question - and it's really important to teach people "in their own language, according to their own understanding".
As an example of what I meant, we read from 2 Nephi 4 and talked about what it says about Nephi. We summarized "the things I have seen and heard" (v. 16 - his visions and what he was told in them) and then looked more closely at vs. 17-19, in which he describes how he feels about his "natural man" tendencies and actions. The wording is quite extreme, and it is worth pasting here (with the words we discussed in more detail bolded):
O wretched man that I am! Yea, my heart sorroweth because of my flesh; my soul grieveth because of mine iniquities. I am encompassed about, because of the temptations and the sins which do so easily beset me. And when I desire to rejoice, my heart groaneth because of my sins.
This paints a picture of serious internal turmoil - and, taken literally without the help of the visionary background, it even could be seen as describing a bad man. That is interesting, since we Mormons tend to describe Nephi in such glowing terms that he comes across as almost perfect - an ancient superhero of sorts. I mentioned how we have the "good parts version" of prophets in our scriptures (and I explained the Princess Bride reference to them) and how we might buy into Nephi as superhero without weakness if we didn't have 2 Nephi 4. (I told them that if Nephi was alive today and talking with a therapist, he might be diagnosed with depression or bi-polar disorder or something like that - and he might be given medication to control his condition.) At that point, our Bishop, who was sitting in on the class, made a really profound comment that led perfectly to the conclusion I wanted to make. He said:
I see that man every day when I look in the mirror. I know what he meant in these verses and in the following verses about knowing in whom he trusted.
I then told the students that we tend to treat our modern prophets and apostles in the same way - overlooking or ignoring their weaknesses and mistakes and talking about them as if they were flawless. I reminded them once again that Joseph Smith was the most chastised person in the D&C, and we talked about his reference to himself as a "rough stone rolling" and what that means. I mentioned how much we tend to talk about Joseph the same way we tend to talk about Nephi - and how hard it can be for some people when they finally realize that Joseph wasn't a nearly perfect superhero.
I told them that if any of them ever was in a situation in their life, at any point, where they felt wretched and were grieving and groaning because of temptation, iniquity, sin or simply feeling like they didn't measure up to what they felt they should be, I hoped they would remember these verses and this lesson and not let themselves lose hope.