ALL of the Eastsheens are the SAME cube - just with different packaging and price points. That narrows the selection down quite a bit - your choices are really a Rubik's 4x4x4, and Rubik's Studio 4x4x4, and an Eastsheen 4x4x4.
If you go with the Eastsheen, just decide how much you want to spend (anywhere between $13.99 or $57.99), and pick the cube that goes for that price. (I know, I know... tough choice).
I've never owned a Rubik's studio cube, but I've seen it get mixed reviews on the forums. I understand it's sturdier and more solidly built than a regular Rubik's cube, but I also understand it's stiffer and harder to turn. Since you mentioned your purpose was for speedcubing, it's probably not your best bet. It's also considerably more expensive.
The Rubik's version - if broken in well and lubed - will eventually spin pretty well, but it's not great for speedcubing right out of the box. It's mechanism also tends to be unstable - it can get so loose that it pops on you all the time.
The Eastsheen version spins like a dream right out of the box, and mine have never popped. But I am told that they don't last as long as the Rubik's versions.
Also, the Eastsheen is considerably smaller than the Rubiks. For me, this is an advantage because I have small hands and my wrists start to hurt after too much cubing with the Rubik's 4x4x4. If you have bigger hands you might prefer the Rubik's version - I prefer the Eastsheen.