I'm trying to decide between 3 bikes: Specialized myka sport disc 29 or 26 inch, trek cali (base model), or trek Skye SL. My budget is between 500-700.

here are the specs for the specialized bikes:

FRAME

Specialized Myka sport disc 29 and 26 (both the same except for size)


FORK- SR Suntour SF13-XCT-MLO 26", coil/MCU spring, 1-1/8" steel steerer, mechanical lock out w/ preload adj., 28mm stanchions, post mount disc, 80mm travel
FRONT BRAKE- Tektro HDC 300, hydraulic disc, dual piston, Lightwave style rotor, 160mm
REAR BRAKE- Tektro HDC 300, hydraulic disc, dual piston, Lightwave style rotor, 160mm
BRAKE LEVERS- Tektro, hydraulic lever, women's shorter reach lever
FRONT DERAILLEUR- Shimano Altus, 8-speed, 34.9mm clamp, low mount, top-swing, dual-pull
REAR DERAILLEUR- Shimano Acera, 9-speed, SGS Cage
SHIFT LEVERS- Shimano Altus, 8-speed Rapidfire plus, SL type w/ optical display
TIRES- Specialized Fast Trak Sport, 26"x2.0", wire bead, 40TPI


Trek Skye SL

Front suspension- SR Suntour XCM w/30mm stanchions, coil spring, preload adjustment, hydraulic lockout, 80mm travel (13"), 100mm travel (16", 19.5")
Wheels- Formula FM21 alloy front hub; Shimano RM30 alloy rear hub w/Bontrager AT-650 32-hole double-walled rims
Tires-Bontrager LT3, 26x2.0"
Shifters- Shimano Acera, 9 speed
Front derailleur- Shimano Acera
Rear derailleur- Shimano Acera
Brakeset- Tektro alloy linear-pull brakes w/Shimano Acera levers
Crank- Shimano Altus, 42/32/22
Cassette-Shimano HG20 11-34, 8 speed


Trek Cali (base model)

Frame- WSD Alpha Gold Aluminum w/internal front derailleur cable routing, G2 Geometry
Front suspension- SR Suntour XCM w/mechanical lockout, preload, custom G2 Geometry w/51mm offset, 100mm travel (14": 80mm travel)
Wheels- Formula DC20 LW alloy front hub, Formula DC22 LW alloy rear hub w/Bontrager AT-650 32-hole double-walled rims
Tires- Bontrager 29-1, 29x2.0"
Shifters- Shimano Acera, 8 speed
Front derailleur- Shimano Altus
Rear derailleur- Shimano Acera
Crank- Shimano Altus, 42/32/22
Cassette- Shimano HG20 11-34, 8 speed



I picked ones with disc brakes. what is the difference between mechanical and hydraulic?