I like using KDE 4.x and for that reason have stayed away from distros like Ubuntu, CentOS, and Fedora. I've heard that Kubuntu does not have the best KDE implementation.
I used OpenSUSE before but it has been awhile and it was using KDE 3.x. I was unable to get a USB cell phone modem to work with it. Also, I have heard that it is unstable and also sluggish/bloated so I stayed away from it. I have used Mandriva for awhile and have really grown to love it but also want to reevaluate. I have had issues with using the 64 bit version. Only the 32bit version works plus I'm having some stability issues at bootup and I'm not sure if it's related to the Intel driver or something with the version of the kernel or KDE 4.2.
I will be using it on an Acer Aspire Timeline 3810T with a 13.3 inch screen. It has a CULV processor at 1.4Ghz and is 64 bit and dual core.
Which would be the better distro to use or is there another distro I should look at?

There's the KDE-based simplyMEPIS which attracts ex-Kubuntu and ex-openSUSE users.

you can check the hcl for each distribution, to see if that phone is supported by default.

Also, the best doesn't exist, it's a personal choice really. Mandriva has it up and down sides (depending on default possibilities), as all distro's have at some point. One distro is better suite for this, the other for that, but in the end they can achieve all the same:
in other words: you can adjust everything in a linux system, that's the beauty of it. Changing configs, recompiling kernels so it supports what you want, compile modules (drivers), etc etc etc etc

You are in control of linux, Windows is in control of the user.
That's the philosophy behind the two OS'es. As you can clearly experience when using them both.