John,

In getting ready to hang the drywall for my basement and just need a
few clarifications. I thought you were supposed to butt the sheets
together but I have been reading some articles that say to leave a
1/8" gap between them, which is correct?

I think the gap is recommended, but I didn't spend much time doing that
when we installed our drywall. Unless you really get fussy about it,
you'll probably have a small gap between sheets anyway, without trying.
And it's not the end of the world if they butt tightly.

I was planning on hanging the sheets vertically, do you really cut the
number of seems in half by going horizontally?

In theory, taping a horizontal joint is supposed to be less work and less
visible than vertical joints.

Having said that, I prefer to hang sheets vertically, especially if
you're working alone. You can just set the sheet against the wall, and
use a lever to push the sheet against the ceiling with your foot while
you screw it in place. That's much easier than trying to lift a sheet
horizontally, and hold it up against the ceiling while you screw it down.

Also, you can usually use smaller (lighter) sheets vertically, since a
horizontal sheet would typically span the full wall width without a seam.
End butt joints are much harder to hide than the tapered edges on the
sides of a sheet.

Yes, you'll probably be going up and down the ladder to tape a vertical
joint, but I didn't find it to be that big of a deal (and we had 14 feet
high walls in a few rooms). It's good exercise and you'll usually need to
climb the ladder to do the ceiling corner joint anyway.

I drywalled the ceilings and walls of our garage, and our entire 1500
sq/ft house. All sheets were hung vertically, and though I'm a complete
amateur, you would find it nearly impossible to point out the joints now
that it is painted.

Take your time, use fiberglass mesh tape on all seams, and apply three
coats of joint compound using progressively larger knives (6", 8", 12").
Hold the knive mostly perpendicular to the wall, as you can make the
knive bend into a depression if you hold it flatter. Let each coat dry
thoroughly before applying the next (usually overnight). I use a special
corner knife for the corners. If you run into a particularly challenging
spot, don't hesitate to apply a fourth or fifth coat to achieve best
results. When sanding, be careful not to oversand and create a depression
(you can always apply another coat if that happens). Prime with a pva
primer, then paint.

The walls are 14.3 x 23.7 and 9 feet high

Order 9 foot sheets from a drywall supplier, have them delivered, and
hang them vertically. Order a few extras in case you make mistakes
(cutting holes for outlets etc.) or sheets get damaged. It's usually
easier to hang a full sheet over door/window openings, and cut the extra
away with a drywall saw after the sheet is fastened down.

If your walls are still open (i.e. no insulation yet), I recommend taking
pictures of all the walls before insulating/drywalling. Believe me, a few
years from now you will be thankful to know where a wire or pipe is
behind the wall.

Good luck!

Anthony