Have you ever tried to paint a wall with texture? It's a huge pain, and takes a lot of paint. Something to consider...

Ceilings should be smooth unless you live in the southwest. Be careful if you attempt to remove popcorn ceiling spray, if it is old it could be hazardous to your health. Have it tested first.

In CA all the houses are basically knockdown from what I see. I see lots of smooth in the rest of the country. Each has it's issues. Flat shows every flaw. Textured hides a multitude of sins. The main issue with painting tex, at least what I have come across, is that the builder only lightly sprays the white paint on the walls directly onto the texture- which is joint compound material. This is very absorbant so when you do your own paint, it take like 5 coats and you still get little spots of white. And the higher your ceiling it seems like the builder sprays even less paint. My 25' ceiling barely had any paint on it. But each time I change the room colors, like kids rooms, it takes less coats. You need a thick nap and either get an electric paint roller or keep the roller nice and wet. Do several light coats. You will be frustrated trying to do two thick coats.

If you plan to wallpaper flat is obviously better. I have finally accepted the fact that any textured wall needs three coats of joint compound to smooth out. I tried using that thick liner paper- horrible. The edges do not lay flat, so not only do you see the lines where the liner matches up, it is all lumpy. Three coats of compound is literally the only solution and a 5gal costs about $10.

Patching is not difficult if you take your time. I have moved a light fixture and because I took my time blending you cannot see the original hole.

So there are issues with both. Flat may be simpler but some do not like the look of flat. Often times folks will do a venician plaster look to take a way from the flatness. But that is a whole other topic.