u should be able to ride all of them but in case u want to choose between the bikes u should go through the following....
CR125
FEATURES
Good Points: very reliable motor. it has the best frame, very durable and it is very quick and fast.
Bad Points: Kind of heavy, can't over rev it to much, once you feel it shaking and it sounds really loud, you got to shift.
General comments: The Honda CR125 is a great bike for intermediate riders, hardly needs maintenance, good for jumps, feels great through whoops, handles well, turns on a dime. I recommend you buy tube slime or a better tube, because it really punctures easily.
KX 125
FEATURES
Engine
•Reshaped combustion chamber
•Higher compression ratio
•Piston ring locating pin moved to rear of piston to evenly distribute ring pressure at front of piston
•Improved KIPS design provides better seal to piston
•Reshaped expansion chamber provides more power and is recessed from rider's leg
•Two-stage reed valve increases low-end flexibility, prevents over-flexing at high rpm
•New carburetor settings
•Straightened air duct and carburetor holder
•Revised ignition
Durability Improvements •Increased clearance between connection rod big-end and nylon crankcase "stuffer", increasing lubrication
•New connecting rod made of stronger steel
•Left crankcase casting is more rigid, clutch cable-to-case now sealed with an O-ring
•Stronger radiator side brackets help prevent deformation
•Engine bolts made of stronger steel
•Stronger exhaust silencer mounting brackets and stronger igniter bracket
Chassis and Suspension
•New UNI-TRAK(r) rear suspension link is anchored on the swingarm, drastically reducing front-to-back movement of the shock, resulting in smoother, more stable action and improving traction at the rear wheel when accelerating
•Forged, one-piece actuating rod for UNI-TRAK replaces two-piece extrusions
•Frame construction is lighter due to suspension loads being absorbed by the new 20mm longer swingarm
•Upper cross tubes and engine mounts are modified to idealize stiffness balance for enhanced cornering performance and improved durability
•New semi-sealed cartridge fork provides better damping and fork action
•More compact front axle brackets feature new clamping method
•Adjustable rubber handlebar mounts help reduce vibration
•Flatter seat improves riding position and raises seat height 8mm
•Textured slip-resistant seat cover helps keep rider in place
•Foot pegs raised 3mm to improved the riding position
•Rear brake pedal pivot is mounted 11.5mm higher for increased ground clearance
•Fuel tank uses oblong mounting hole for easier maintenance
SPECIFICATIONS
•Engine: Two-stroke single cylinder with KIPS(r)
•Displacement: 124cc
•Bore x stroke: 54.0 x 54.5mm
Cooling: Liquid
•Carburetion: Mikuni TMX38x
•Induction: 6-petal carbon fiber crankcase reed valve
•Compression ratio: TBA
•Ignition: Digital CDI
•Transmission: 6-speed
•Rake/trail: 27.5 / 3.9 in.
•Suspension adjustments, rear: Spring preload, 16-way compression and rebound
•Wheel travel, front: 11.8 in.
•Wheel travel, rear: 12.2 in.
•Tire, front: 80/100-21 51M
•Tire, rear: 100/90-19 57M
•Brakes, front / rear: Hydraulic disc
•Dry weight: 191.2 lbs.
•Fuel capacity: 2.2 gal.
RM 125
FEATURES
* Flat top piston (lighter) for more bottom power via closer squish area.
* Cylinder all new with non-bridged exhaust port with exhaust sub-ports.
* Transfer or boost ports have greater volume from the cases up for bottom hit.
* Went from a two-piece exhaust valve to a large single valve and small sub-port valves to make engine respond more like 250F.
* Long-rod (8mm) like '03 factory bikes for better hit.
* Crank inertia is increased 6.5-percent for more roll-on.
* Airbox like '03 250, with intakes in sidepanels for cool air, and straightened air boot and manifold.
* Shift stopper spring stronger and detents more solid like RM250 for more positive shifts.
* Seat 10mm lower than '03.
* All hardware and many detail items stronger and better quality.
* Quick-adjust clutch perch for first time.
* Suspension has refined valving front and rear, with redesigned compression valve for better hook-up with new link.
* Linkage rods attach to swingarm rather than frame.
* Redesigned front brake master cylinder and Honda-style hose routing for aftermarket performance stock.
* Wheel drag is reduced via a floating front disc and new carrier in rear.
* One-piece rear master cylinder/reservoir.
YZ 125
FEATURES
•Improved pull down low
•More top-end power
•Better gear spacing
•Great ergonomics
•Very stable at speed
•Suspension offers an improved feel over '02