Having owned both a 350 twin and Four I'd say the 350 Four is a better bike, less maintenance, very smooth, but I prefer the 350 Twin. It
s much much more fun. It's easier to work on, you can rebuild the carbs yourself, troubleshooting is easier and riding is more relaxed. In many ways the twin is like a super reliable old fashioned motorcycle while the 350 Four was simply an early modern motorcycle (the first of the UJMs along with the 500 and 750 Fours).
On the 350 Four it seemed like I was always trying to ring every last drop out of it, it was always a race to get anywhere and was disappointed at how slow it was. On the twin you're more inclined to simply put around and enjoy yourself. It also has a front disk which is a pain in the ass compared to the simple drum of the twin. Nothing went wrong when I owned it mechanically besides the disk brake, but I was constantly terrified it would and I'd never find parts for it, let alone be able to fix it!
"You've never heard of the Aluminum Falcon? Its the bike that made the Blue Note run in less than 2 minutes. She's fast enough for ya' old man."