Painting contractor here, but opinions vary.
I am decidedly not a big fan of "deck-over" type products. They are basically putting a thick coating of plastic on the deck. Any imperfections in prep, and often standing water (or snow) will cause them to peel right back off. They are a last-ditch effort for people who really should be replacing the boards.
If your deck was previously painted, you will have the film visibly coming off. You have to remove any that is not adhered completely. Get any obvious areas with a scraper, and then powerwash with a mild bleach/detergent solution (like Jomax) to get off any mildew or dirt. From there it is a good idea to sand all areas with some heavy grit (60-80) sandpaper just to feather in the edges of the missing paint and providing some roughed areas to help the next layer adhere. Once you've done this, one coat of solid stain on uprights and two coats on horizontal areas following the manufacturer's dry times. I use Sherwin-Williams commercially, but actually like the less expensive (5 year) Behr solid stain from Home Depot. In most cases, coatings will not hold up past 5 years or so in most areas on horizontal surfaces- pretty much no matter what product you use. Sun, snow, rain, whatever the climate, it will deteriorate.
If there isn't much peeling, it was probably a solid stain, not a paint. While there isn't much of a difference between the two, stain tends to be lighter bodied and soaks into the wood better.