I guess I have a contrary opinion on the solid redwood stain. I have a small vacation home in the Brazos Cliffs area of New Mexico. Elevation 7500 feet, 90 inches of snow in an average year, monsoon seasons for two months in the summer and outrageous doses of UV year round. I am just a Walmart/HomeDepot kind of guy. When I bought the property 6 years ago the detached garage/shop had very heavy discoloration of cedar siding below the snow level, was cracking badly and looked on its last legs. I ran a horizontal 1x4 of cedar around the structure just about a foot above snow line and stained with then available 3/5 year Behr semi-transparent cedar above the 1x4 and Behr solid cedar stain below. Currently the solid stain looks as good as when I put it down. The semi-transparent is also in not bad condition except for the new cedar rail I installed on the west-facing side...not bad, and I really like the color pairing of solid and semi-transparent. At the same time we stained the deck and rails with just semi-transparent 3/5 redwood. At year three, the deck was clearly in need of staining, but I put it off until this spring from general laziness. The vertical surfaces at year 5 or 6 were in need of restaining from upclose appearance, but looked fine from the road. The product seemed to me to meet its projected longevity goals as advertised. This spring I decided it was time to stain the house and again used the Behr stains. The house, snow-shedding deck cover columns and beams, and vertical supports of the railing for 800 sf of deck are in semi-transparent redwood 5/8 yr and the deck and horizontal hand rail surfaces are in the solid 10/25 yr redwood stain. I really like the color combination and expect long service from the solid stain. We did the 15-min surface treatment prior to staining, but nothing special beyond that. No bubbling, no stickiness, no problems at all. Yes the solid is a bit orangey, but the combination of semi and solid looks justs as attractive as we found it to be on the garage years before. Obviously color tastes vary, but I think using solid stain in high stress applications can be a great labor savings in the long run. I wouldn't want my whole house in the solid stain color, but with the house freshly stained in semi-transparent, I find the solid stain to provide a classy-looking trim effect on the horiontal features of the deck. In a couple of years I plan to stain the garage redwood too, but there is no rush--the stain is holding up just fine for now.

"Both products are water-based." That is the essence of your problem. If you are determined to use a water base product don't waste time stripping off the old Behr junk. It will not improve the performance of the new product. Sand off the loose stuff, give the deck a good cleaning and rinse. If the new product is better than the Behr, you will be stripping it off in a year or two after it fails. Make sure you understand that water base stain is really just paint. It forms a film on the surface (regardless of what the label claims it "penetrates into the wood" total baloney) same as paint. "Also, we are considering painting the floor boards a different color from the railings." So it sounds like you already understand the water base product is paint. Why not get actual paint? I am sure the BM paint store has some paints designed for exterior wood high traffic areas. I would choose an earthy, neutral color for the deck boards to blend in with the outdoors, and match the railings to the color of the house/trim. I think you will get way better performance with a high quality exterior paint designed for use on porches and decks.

Be careful. Oil based stains have changed. Read the label and follow the directions. Do not ignore the recommendations or you will invite a mess. Solid color stain that is oil can probably be applied with no primer thereby eliminating a step, whereas a solid acrylic will probably need a primer. However, the acrylic will hold up in the sun better over time, providing it is properly applied. Paint will have a heavier film build to it and maybe even some sheen, like a satin? Personally I'd be happier with the look of stain, which is flat.

I don't care for solid mid tones. I stained my old pergola wood a solid dark brown (Cabot stain), and it looks great. The texture (relief) of the wood grain shows but not the pattern of the wood grain. It does look more like paint, but being so dark, it can be confused for very dark stain. I think mid tones would look more like paint. I stained mine to match the trex woodland brown composite I used on the adjacent deck. It worked well. I did not stain decking or any horizontal surfaces - just posts, pergola slats and faschia. Decking is a different animal. I would call the manufacturer and ask them for their recommendations for specific color and product.