It all depends on what you want to do with it, patio, trailer mounted, big parties, etc. There are a number of great vertical units out there, Backwoods, Stumps, Pitmaster along with numerous traditional horizontal builders making their own versions of uprights or verticals. If you plan on comp cooking or just want a more hands off approach (ie: sleep), most people are going with verticals. Minimal fuel, long cook times, insulated, heat control, all are +'s. Depending on the size you buy and the ability you have to move one around, there are some large verticals that do as much if not more meat than a standard patio offset. Most are ready to hook up DigiQ's or BBQ Gurus within minutes for even easier temp control. I use an offset RF trailer rig that I built earlier last year and love it, but if I ever decided to really get out and cook maybe 12-16 or more comps a year and travel a bit more (currently cooking 6-8), I would probably be leaning towards a vertical unit. Pack all your stuff into a 12 foot cargo trailer including a Backwoods Chubby or 2, prep tables, a sink and have a place to sleep at night in the trailer. Sleep is critical when your traveling all day on Friday, cooking all night and all day Saturday, then turning around Saturday night headed home. I have everything I need on my trailer rig, wood storage, propane burners, tool box for all my supplies and tools, a sink wiith running water and of course the pit, everything I need to travel and cook, but it's never going to be as easy as the verticals.
I think out on the comp circuit, it's probably nearing a 50/50 split on horizontal to vertical units. Of course there are also a lot of pellet cookers (which to me is just flat out cheating!), some UDS's,
WSM's and any number of other units being used as well. Personally, while it may seem like I am defending verticals or uprights, my heart is with the traditional offset (especially since I built and own one), there's just something about black steel, smoke and that offset look that screams "Good Q cooking here!"
Vertical for easier heat control CON; less cooking area. Take a look at the Pitmaster Vault (some of the finest looking rigs made today! They also offer an offset, the Sniper.) or the Stumps stretch, you can cram plenty of food into those bad boys.
what makes it easier to control the heat vs a horizontal with or without a side box? Smaller fire, charcoal baskets that will burn in a "S" shape, insulated, water pan acts as a heat sink helping with a more even temp.
Still most serious Q'ers out there use horizontal smokers. For the Joe American, that is 100% correct. One of the main reasons is the availability of old tanks, pipes, etc. We all know someone who can weld or can help with the construction of a traditional style pit, but very few that have ever encountered an upright. I don't know if "serious" is the word, maybe traditional...that fits. Here in Texas, especially out in the Austin area where all the famous Q spots are, they pretty much all cook on offsets, not always steel, but brick and mortar with a fire to one side. Still offset cooking.
I'm sure CroMag can add some more positives and negatives since he once was an owner of a vert. Hope some of this info has given you some insights.