I was right where you are at about 6 months ago. I researched, asked tons of questions, got LOTS of advice from the Brethren, and basically I came to the conclusion (through all of the advice) that the UDS was the way to go.
My reasoning was that it seemed to me that there were a lot more design concerns with a horizontal smoker (diffusers, tuning plates, vents, insulated firebox, airflow, etc.). I read a lot of posts where people had to do a LOT of adjusting, modification, and farting around with their offsets to get them to perform the way they wanted. All the reading I did on UDS's suggested that they were farily foolproof as long as a few details were followed.
I agree, still, with your reasoning that the a 55 gal drum on it's side yields more cooking area. Being a newbie, I just didn't want to deal with the headaches of not getting it right the first time.
I decided on building the UDS. I've got 2 racks in mine, and it'll hold quite a bit of grub. I still want to have a horizontal offset one day, but for now....I'll continue to perfect my methods and keep on learning on the UDS. Right now, bewteen my 2 smokers and my 2 grills, I'm good.
I don't know your level of BBQ experience, so I could be just talking out of my arse here, but I'd suggest the UDS as a GREAT starter smoker (if that's what you're after). Heck, for that matter, I scored my ECB for $15 from CL and with a couple of cheap mods, I did just about all my initial 'Q' learnin' on that thing. By the time I did finally build my UDS, I was off and running right away. Lots less "tinkering" to keep the fire consistently rolliin along.
Anyway.....good luck on whatever you decide. I'm just saying that for the ease of construction, and overall hassle-free performance, it's hard to beat the UDS. ESPECIALLY when you're learnin like me.
Big JT's Smokin' BBQ Competition Team
"The path was worn and slippery. My foot slipped from underneath me, knocking the other out of the way, but I recovered and said to myself, 'It's a slip and not a fall.'"