@Michael.A.H: Both are equally terrible if you really do need to read complex PDFs. KOReader makes that noticeably less painful, but I still wouldn't really recommend an eInk device for that.
Kobo's software is not that great, but a bit more customizable.
A Kindle is hands-down much more noob-friendly.
My biggest gripe with Kobo would still be their apparent lack of interest in driving Carta eInk screens to the fullest extent of their capabilities (don't jump from a PW2+ to a Kobo, that'll make you sad, in terms of ghosting & refresh speed).
As mentioned, there are large rendering and performance differences, as well as a number of varying quirks depending on whether you read ePubs or kePubs (you could compare that to the mess that's starting to develop on Kindles with Mobi/KF8/KFX).
If you've never extensively used a (recent-ish) Kindle before, the Kobos are perfectly fine.
I have to admit that I do miss those .8" when I see a PW, though, but it's not that big of a deal if you've never extensively used a 6.8" device
.
If you care about that sort of thing, a Kindle (especially the latest generations) will be scarily less bulky, lighter and thinner, but that doesn't actually deter you from finding a Kobo perfectly usable.
FWIW, the device that I've been carrying around with me everywhere is the H2O, the PW2 stays at home, and hasn't seen much actual "use" (as an eReader that is. As a fun thing plugged in to my computer to poke around in, that's another kettle of fish ;D).