Both are irrelevant. Here are some things to check out before you even start playing. You can play longer on a slant top machine because you can rest your arms on it, while playing the upright machine can be tiring if you're there for any length of time. On the other side of the coin, there sometimes is more glare off the slant tops than you'll get on the uprights. So it may be a decision that only you can make.
Next step is to check the screen images.
And then, of course, there is the machine with the sticky keys. Avoid them at all costs. It can cost you a winning hand. Whether you know it or not, machines have different speeds, with some of the newer machines having adjustable speed controls. You may be a player who likes a fast machine and are annoyed by a slow machine.