I have the Miele 30" all fridge and all freezer, and love them. I essentially approached the problem backwards. I do freeze a lot, and have had an upright freezer in my kitchen or pantry for the better part of 20 years. I would be fine with a freezer on bottom, 36" fridge if I had an attached garage or other easy access place for the freezer. In this house the kitchen was the best place for the freezer, so a prettier full sized upright freezer was part of my remodel plans from the beginning. And having that, I didn't need a freezer in the fridge.

I've seen two 36" over/under units installed side-by-side, mostly before the column units became popular. This is good if you want/need more fresh food storage than frozen, but there's more that can go wrong. Either your fridge-freezer units have one compressor so they share air and sometimes compromise between the two compartments to maintain temperatures, or you have dual compressors with the fresh and frozen compartments on separate systems, and essentially four units instead of two. An all fridge and all freezer are just larger, simpler units.

As to looks, 6' of uninterrupted refrigeration can be a bit...blank. You can use styling to make this better. Some people have cabinetry panels made so that they look like a bank of smaller doors and drawers. Some people put art on the doors. Some use large, decorative pulls. I have 5' wide, tall units, which gives more of a vertical appearance to the refrigeration, with big, honkin' custom pulls that break up the space. Some all fridges (a certain model of SubZero, for sure) have a window, which breaks up the surface and gives you deeper shelves (no door storage.

Another option is to put the fridge and freezer in different locations. Many people find that they don't need both at the same time/place. The freezer might be closer to the pantry storage and the fridge closer to the prep zone, for instance.

Other people have done very clever designs that make the pair with a surround so they look like an armoire, or other kind of contrasting idea. And some just leave them as they are, figuring that appliances should just be and look like what they are.

You haven't mentioned your price and performance preferences. I believe Frigidaire makes freestanding all fridge and all freezer for a very reasonable price. Thermador makes mid-range built-in columns (i.e., integrated all fridge and all freezers in several sizes). The top end are built-ins by SubZero, who make a lot of different sizes and shapes, and Miele, whose columns are the same size and shape as Thermador (same factory), but whose electronics and features are by Miele. Which ones you choose depend on your budget, style and preferences for interior arrangements and features.

If you go to a website like AJMadison you should be able to get a good overview of what's available.

Thanks so much for the great advice.