I have one and prefer it to the KT88 version, but the KT88 version is just as good. It's personal preference really. I like more more mid range and a spongier feel, so EL34's work best for me. The KT88 version has more dimension and an overall bigger sound, but sounds a little papery to my ears. The cleans are a bit better on the KT88 version, and KT88's are more detailed. EL34's are slightly more congested, spongier, and more mid range than KT88's. It's pretty much the general difference between big bottle vs. smaller bottle tubes.
It doesn't matter which version you initially buy because you can convert it by adding a single resistor and a rebias. The resister change won't even affect the warranty. I agree that you should buy the KT88 version if you never got to do an honest side-by-side comparison of both versions, but I'll take it one step further and suggest that you try two things before making the decision to switch to EL34.
First, try swapping the tubes. I tried Valveart KT88's and it gave me a bit less stiffness and a slightly softer, more vintage tone (very subtle difference but I liked what I heard). The stock tubes are Sovtek which are very reliable but to me are slightly harsh. The 2nd thing to try is 6550's which people say is like a KT88 mixed with an EL34. I personally never tried them in my amp, but I have a friend who works for Guitar Center and took a used one in a few months ago that had 6550's. I played it through a 2x12 with Greenbacks and I was really impressed.
If I ever get the urge to try something different with my Shiva 20th EL34, I'll try 6550's. I don't necessarily agree with what people have said about the 6550's sounding in-between EL34's and KT88's. To me they sounded pretty much like 6L6's but with a little more texture. It's hard to describe since I didn't spend more than 1 hour with that amp. I can tell you they're warm and sound very much like 6L6's except slightly bigger but not as big as KT88's. "Texture" is the only way I can describe it as I think of 6L6's as kind of smooth but generally slightly bland mid range. 6550's kind of have a slightly less bland mid range than 6L6's. It's all a very subtle difference though.