HAHA um... I use.... printer-paper sized cardstock. Like. I go into an office supply store and buy a package of store-brand 8.5x11" cardstock that's meant for printing stuff on and that's what all my comics get drawn on. xD I prefer it because it goes through the printer easily, and printing out the sketch in non-photo blue is a big part of my process.
When I was in art school I used 11x17 Bristol Board for all my projects, because at one point that was the Industry Standard, but in comics now that's kinda becoming less and less important.
As far as what type of paper, there's not really any "should" at this point, honestly! You should use whatever works best for your process. I use cardstock because I ink pretty heavily in ballpoint pen but need the paper to go through my printer. Another person might use Bristol Board because they're going to be using india ink on it. My previous comic was inked with micron pens on a piece of normal computer printer paper folded in half because that's all I needed and that helped me keep it a consistent size.
As for what size they should be, the place that will matter the most is if you want to print your comics at some point! It sounds like the two styles of paper you have are in the standard/western size ratio and the manga/digest size ratio, so honestly, whichever one of those feels like a better fit for your comic is the good to use. (I just recently posted photos of these two sizes of book for comparison in another thread if that'd be helpful!)