I would say this:
On 4.4.x, the GPe ROM or CM11/AOKP/PA/PAC is better choices of the simple reason that they can use phablet mode properly. I am talking about changing the DPI to 240 with separate notification/quick settings bars. On the other hand, Settings lacks tablet mode on 4.4.
The problem with the Sony ROM (4.4) is the loss of working notifications when the DPI is lowered. They have modded the notification bar with quick settings, which breaks down when something lower than 320 DPI is used.
On 4.3, the stock Sony works perfectly in phablet/tablet mode on 240 DPI.
I think 240 DPI is perfect for the Ultra, it uses the screen space very efficiently and enables phablet/tablet mode in different apps etc.
On GPe or CM11 et al, 240 DPI works perfectly. It is also worth to mention the fact that most Sony apps are ported and runs great on the GPe ROM. There's no need to use a Sony stock/based ROM just in order to get Album, Movies, Walkman or Sketch etc to work. Even the latest Z2 based versions of those apps run great on GPe et al ROMs.
This fact, paired with the problems caused by DPI changes result in this simple situation:
1. Sony based ROMs are better in 4.3 since they can handle phablet/tablet mode properly.
2. On 4.4, go for the GPe or other non-Sony based ROMs like PA or AOKP.
3. The Sony Edition of the device is more spread and gives more ROM options compared to the GPe one. It is therefore better to get it from Sony and then flash the GPe ROM.
[Xperia Z Ultra C6802][ROM: GPe 4.4.4][Launcher: Nova][240 DPI]
[Xperia Z C6602][ROM: PAC 4.3][Launcher: Nova][240 DPI]