well, my opinion is: what do you want to do?
If you just want a cruisin' bike, or don't like shifting, or want to race other single speed/no gear 4 stroke "x bikes", the x6 is for you.
If you want to eat x6s, 50s, and everything in between, you want an rx 12 or an immortal.
personally, i have an immortal. It sits lower than the rx 12 and has a slightly longer wheelbase (i believe).
but i goes back to what you want to do. If you just want to screw around and stunt and also have the fastest bike on the block , you probably want an rx 12. they sit higher than the immortals and wheelbase is shorter, but have the same motor. frame is the only difference.
The immortal's shifter is simply retarded ( look at people's pics who own one), while the rx 12's seems to be a very good one. However, the shifter looks to hang really low and i would worry about that dragging/grinding or bending as i can knee drag off my immortal and have no problems, however i had to have it extended.
The rx 12, having a shorter wheel base and sitting taller would be a lot easier to wheelie and stoppie. I can wheelie my immortal (and boy is it fun) but you have to pin it in first gear, then hold it in between first and second, then dump it. It will go straight to 12 o clock. sitting down. Stoppies are possible too, however take more effort. if you have the front hydraulic brake it is also very nice. Again, i stoppied my immortal and it broke the rear exhaust mount (something else to get welded).
the immortal, sitting lower helps when leaning off in the corners, and the longer wheelbase will help KEEP from wheelieing. I have literally ground a hole through my kneepads because i had my knee DOWN. (if you've never knee drug, do it. it's awesome). But i can eat 47cc cags and 50s on the immortal. Through the corner's it's awesome, just have to watch your shifting. It WILL throw you off if you rub it just the wrong way. But on the straights it shines. i have an aftermarket rev box (like BIG GUN EXHAUST i think) and it revs to the moon. It's fun to listen to the 2 stroke pocket bikes just screaming then just fly by. but it's the simple things (obviously).
Again... it just comes down to what you want to do. I have heard each of these are extremely fast (it's all the same motor except x6 has no gears)
another one to keep an eye out for is the
x7 4 stroke but they're coming with the lights and turn signals etc etc, as well as kick start (hint: check
ebay)
anyways... this is my opinion. the immortal can have ALL plastic removed in under 5 minutes, while the x6 is a real pain in the @$$ to get all the plastics off. rx 12 seems to be average.
X6 seems to be the cheapest in the bunch, but lack of gears:
Pros: fast! 4 stroke!
Cons: lower cost, no gears
bottom line: good for a racer or hobbyist on a budget
Rx-12:
Pros: Gears, fast, wheelie machine, lots around (easy parts, competition for cost)!
cons: sits high, shifter hangs low
bottom line: excellent bike for stunting or just pleasure riding
Immortal:
Pros: gears, fast, sits low, comfortable, hard to find (you'll be the only one with one)
Cons: hard to find parts/plastics for (however engine is the xr50 replica, so honda dealerships are all over the place), fork seals problem (currently)
bottom line: excellent bike for a racer or hobbyist.
but, once again... what do you want to do?