Should I smooth out or apply knockdown texture after popcorn ceiling removal?
If you're approaching this from a diy perspective, smooth ceilings require more skill and are more time consuming, as the mud job has to be better. (Think 2/3 coats with sanding between.) The mud job under texture can be a little more rough and still produce decent results. (Chances are you could do one touch up coat after scraping and be done.) Texture requires 1/2 day rental of a compressor, hopper, and mixing drill. The rest of the necessary tools are similar. Texture isn't hard to shoot, but you will need to plastic off all walls/floor. (Do this before you start the rental clock ticking.) One of the joys of texture is that if you do a really crappy job, you take it off before it dries and do over. 

So, long story short, texture easier and faster but slightly more expensive in terms of pure outlay. (But not if time is money -- then it's much cheaper.)

If you opted for smooth ceilings with textured walls, that'd look fine.