Survivability for CSM Daemon Princes: Which Mark to use?

As I was drawing up various lists for a 500 point doubles tournament (this post shows my final list I decided on), I posted asking for some advice. My original plan was to take an unmarked DP to save points, but CrusherJoe posted a really interesting comment:

Hmmm...I don't have my CSM codex in front of me so the points might not add up...but I don't like to field a DP without a mark....preferrably the Mark of Tzeench. You could then -- again, if you could find the points, which at 500 points is probably not going to happen -- give him a power or two. Either way, with the MoTz and the wings you've got a flying, 3+/4++ CC rape-monster that is good against infantry, light armor, heavy infantry, MCs, HQs...you get the idea. :)

This got me thinking over the weekend, as to which Mark would give me the most survivability on the may Daemon Prince (I'm rubbish so my DP normally dies easily). Traditionally, I've always used the Mark of Nurgle (+1 T) as my army is all Nurgle, but having recently purchased a cheap Necron C'Tan model to convert I've got an opportunity to create a different prince. So, how much more survivable is the Mark of Tzeentch (+1 to the invulnerable save) vs the Mark of Nurgle? I had a gut feeling I knew what the answer would be (depends on what you're hit with), but I decided to run some maths.

To re-cap, a Daemon Prince is base T5 with a 3+/5+ save. Mark of Nurgle make's it T6 with a 3+/5+ save, Mark of Tzeentch T5 with a 3+/4+ save.

Let's look at each Mark being shot with a different weapon and the likely hood of suffering a wound (basic maths being chance of wounding x chance of failing a save, with the answer rounded off):

Shot with a bolter (S4, AP5)
Mark of Nurgle = 0.166 x 0.33 = 5% chance of a wound
Mark of Tzeentch = 0.33 x 0.33 = 11% chance of a wound

Shot with hotshot lasgun (S3, AP3)
Mark of Nurgle = 0.166 x 0.66 = 11% chance of a wound
Mark of Tzeentch = 0.166 x 0.5 = 8% chance of a wound

Shot with a plasma gun (S7, AP2)
Mark of Nurgle = 0.66 x 0.66 = 43% chance of a wound
Mark of Tzeentch = 0.83 x 0.5 = 41% chance of a wound

Shot with a melta gun (S8, AP1)
Mark of Nurgle = 0.83 x 0.66 = 54% chance of a wound
Mark of Tzeentch = 0.83 x 0.5 = 41% chance of a wound

Shot with a demolisher cannon (S10, AP2)
Mark of Nurgle = 0.83 x 0.66 = 54% chance of a wound
Mark of Tzeentch = 0.83 x 0.5 = 41% chance of a wound

Once you get to the high strength, low AP hits, MoT is better as you're being wounded on 2+ regardless of whether you're T5 or T6 once the strength of the hit gets to 8 or higher. At low strength (see the hotshot lasgun for an example) MoT is better as it's a 6 to wound both T5 and T6. So between S4 and S7 regardless of AP, MoN is generally better. Once you get over S7, MoT is superior, as the advantage of +1T is lost.