Horizontal vs Vertical offset smokers

I had a Klose 20 x 48 direct offset with dual racks. It was awesome. It had four distinct heat zones that you could use to your advantage when cooking different things. If you were trying to cook one thing, there was a bit of busywork involved in rotating the meat to cook everything evenly.

It got destroyed in a move and I replaced it with my current pit, a Shirley 24 x 60 reverse flow. This pit is amazing. The craftsmanship is incredible. It holds even temps across the grates (+/-5°). The even temps allow me to cook large quantities without much interference. It also has a warming oven over the firebox that comes in handy.

Back to your original question: I am not a fan of verticals, because I don't like the upper meats dripping on the lower racks (e.g. chicken dripping on ribs). The advantage is that they have a smaller footprint if you are tight for space.

If you do get a horizontal, I highly recommend the 24" over the 20". It allows you to put briskets and ribs in perpendicular to the racks instead of lengthwise.