Both! But I almost always use a solid or wood grain paper for my background. Maybe a subtle, small scale pattern if I want to step outside my box. I love patterned papers in smaller doses, but my layouts look too busy if I use them for my background. (My gallery probably shows more patterned backgrounds than I really use, since I sometimes make an effort to use patterns more in CT pages. But if you include all the unposted layouts that I just do for myself and my family, my backgrounds are overwhelmingly solid.)
I do both. I just take the paper I like best or the one that matches the best with the photos I'm using. I really like tone on tone background papers. They're my favorite.
I'm pretty sure I'm 50/50 on this..... I like prints, but not super busy ones (I like woodgrains, small dots, small stripes with only 2 colors etc), but I also use a solid black or white a LOT!
Probably prints. I used to only use solids, but now I try to stick with solids for areas that I'm going to journal. In a year, that might change as I'm always changing my scrapping style!