I am not quite understand why you want these 2 machines. But anyway, There are few reasons that I will go for the nMP
A) I need thunderbolt. (Your 4,1 can get the USB 3.0 by just $20, plenty of graphic card that works in the 4,1 has HDMI. So only thunderbolt matters)
B) I love the nMP's form factor. (It seems this is not the fact, otherwise, you won't consider the 5,1)
C) I need a higher CPU single thread performance Mac. (Unless iMac does not fit your other needs, the iMac obviously do better in this area).
D) I need Apple care
For the 5,1
1) I want to upgrade to dual XEON, and I don't want to deal with de-lidded CPU. (This is the only reason I will go for the 5,1).
In fact, since you already own a 4,1. It will make much more sense to just flash your 4,1 to 5,1. Install a W3680, and whatever the GPU you want (my dual 7950's performance is very close to the dual D700, except I only have 3G VRAM per card. However, I can use both card for display, unlike the the nMP only one card to drive all 6 ports. Therefore, I have 6G VRAM for display, same as the D700).
$500 should be more than enough to get a W3680 + 2x 7950. Another $300 to get a 1T 850 Evo, plug that into the lower optical bay. It's will cost you less than $900, but you will get a much better machine than that 5,1.