I can tell you about my experience with an adjustable holly hock retainer I just finished with...(the middle spreads out to spread out the roof of your mouth.)
We had to spread out the roof of my mouth, because I have terrible tongue thrust at night and I had pushed my upper and lower teeth forward, which caused the roof of my mouth to curve forward. Anyway, we put on an adjustable hollyhock for the last 6+ months. Here is my experience with all day retainers...
Yes, I lost weight because like others said, I didn't want to have to eat a snack, then brush my teeth and put my retainer in. It was a hassle. I stopped drinking all soda and juice , because of the same reason...
But after a few months, it was REALLY hard. I started leaving it out sometimes after a meal, because I thought I wanted a snack soon. Or I would want to sip on a lemonade. Then I would forget to put it back in. Then at night my teeth would hurt awful again, like after a tightening.
The retainer would smell bad if I didn't wash it with retainer cleaner every day. - BUT, it totally did the job. It straightened my top teeth pretty well, while spreading out the roof of my mouth. Honestly, I think the pain is the same whether you are doing it with a retainer or braces. I mean, you are moving teeth. It's gonna hurt. But not forever. I would just need to eat slower and smaller bites for the first 3 days. I didn't need pain killers.
My dentist said no matter how you do it, it takes about 6 months to straighten teeth, then another year for those teeth to settle. So, 18 months is VERY normal amount of time to move and straighten teeth. Longer if you have major issues.
If you go with invislign, make sure you wear the retainers for a year after to help those teeth settle.