you can see the horizons in person & kick the tires at bass pro shops.
This. Even if one is relatively far away, it might be worth looking. Sizewise, 20 inches is smallish. A 16 inch is fine for... um, well, I'm not sure really.
Unfortunately my bass pro shop only had 16inchers, but if you have one around your place you can check their stock. Unless you have seen a 20 incher in person and found it acceptable, you might be surprised by how small it is. When cooking you may not be able to easily use all the space because of hotspots, so keep that in mind. If you do fill it up to capacity, you just have to accept that you will be rotating meat. However, Id think that you could easily do a packer and a butt at the same time on a 20 incher without having to constantly rotate.
Another good reason to put eyes on one, is to make sure you are ok with the quality. My bass pro shop had a pretty ragged floor model with lots of weld spatter and big door gaps. That is not to say they are all like that, though. Their bigger models may be of higher quality, or the one i saw might have been a bad one. If they have stock, you could maybe look through for a clean one.
Like others have said, think carefully about how you would use the vertical section. That horizon model is on the expensive side. If you can play at this pricepoint, there are some other real decent manufactures to consider. If it were me, I'd put my money on more horizontal surface, which can be used for more capacity, or for doing something real low and slow when the occassion arises. Keeping in mind that there are other cheaper options to keep food warm, besides using the vertical as a warmer.