Some people are going to have problems upgrading OSs one way or another.
And I honestly feel that those who have the major problems are the ones who's hardware is teetering on the recomended limits (old hardware), or have modified their system at the core (thus having a problem when doing an upgrade install).
You seem to have very new hardware and I really don't think you'll have much problems really...
I just strongly recomend backing up your user folder like to another drive, partition, other computer, or external storage before you do an install...
If storage space permits - I'd also recomend backing up your Applications folder, I had such a painless and smooth transition by doing that - and just dragging the programs I wanted back into the new Applications folder. I only had to re-install 2 of them.
Once you've backed up your data to a different drive/partition, it's a good idea to do a fresh install so you'll have the least possible problems.
Oh, and in case your Macs came with MP3s (our new one at the office didn't - but the one I bought a year ago did, weird), you might want to back those up too.
Good luck!