I'd certainly pick KDE over Gnome these days. I don't think there's a KDE distro that I'd rate below Gnome in its current state, Gnome 3 needs to mature and I really hope the Gnome Foundation look at things like MATE and Cinnamon and understand that a lot of users want something more like an evolution of Gnome 2 rather than the monster that is vanilla Gnome 3. Gnome 3 strikes me very much like the foundation's idea of what they think we should be using.
KDE is a more mature desktop than the Gnome 3 series, it has a lot of customisation options and I feel KDE have an understanding that users want choice in the layout of their desktop and a good underlying usability. I don't feel the latest KDE desktops are quite as greedy on system resources either, which was an issue I noticed with older distros.
The thing I find with Kubuntu is it does look a bit generic, it's not as pretty as other KDE distros so it does feel a bit more vanilla. I didn't really take to the Oneiric base even when it came through with Mint. It just didn't play as nicely as I'd hoped with my system. But I've been using the 12.04 Kubuntu alongside other more minimal distros like Bodhi (E17) and Xubuntu (XFCE) and it compares very well. Kubuntu still doesn't have the eye candy that others have, but it's been stable and performed well for me. I'm expecting the new Mint KDE is going to be fantastic, because I'm really happy with both Xubuntu and Kubuntu 12.04, my partner uses the main Ubuntu 12.04 and he's getting on fine with it too. So the base Ubuntu seems to be a good one. I've got the partition all sitting ready for the new Mint, the minute that testing ISO pops up it'll be in there lol.
OpenSuse is very polished looking, however I found it rather over-complicated compared to other KDE distros. There was certainly a learning curve I wasn't expecting as a seasoned KDE addict. I don't usually have to read the instructions y'know?
It was rock solid and reliable, I just didn't warm to it as I hoped to.
The new Netrunner is due to release very soon. I tried the RC and it's very impressive. The default software choices were good, it certainly looks good and I didn't find any glitches or problems with it even as a RC. I was kinda wondering if Mint might opt to base the next KDE on top of Netrunner given the new relationship they have with Blue Systems, but we'll have to wait and see how things go. Ubuntu 12.04 seems solid though.
There are also some newer distros you might not have tried. ROSA have a new KDE distro released recently. It's based on Mandriva, and I found it very user friendly, professional and nicely put together. ROSA have added some useful modifications of their own. I think it's a real "one to watch" as it develops.
And if you want to compare Gnome and KDE on a Ubuntu base, you might want to play around with a French distro called Hybryde. It only seems to have a 32 bit version just now, but basically you can switch between a whole range of desktops without logging out. Literally you click on an icon, open the menu to choose your desktop and it switches over. So you can be using KDE one second then flip into Unity the next. I'd certainly recommend it while you wait for the new Mint KDE because it's Ubuntu 12.04 based. It just struck me as a fun distro for those of us who like to change desktops all the time heh. But as you mentioned choosing from Gnome/Unity to KDE I thought hey, you could have them all
It does load up in French at first though, so you need to get the English language packs in there. But it's a fun distro even if you just play with the live disc.