Front suspension and brakes
Some big differences here
964 uses steel uprights, normal versions have axial mount small body 4 piston calipers w/ ~98x44mm pads. RS and turbo use steel axial mount uprights w/ either medium/ large or large(aka Big Red) size, 4 piston calipers using either ~132x48mm pads for the medium large and ~132x58mm pads for the large size calipers. n/a use 298x28mm rotors, turbo and RS use 322x32mm 1 piece rotors
993 use aluminum radial mount uprights on all models, RS & GT2 use special versions w/ short steering arms to deal w/ bump steer on lowered cars. brakes on n/a models use a version of the medium/large 4 piston caliper using the same ~132x48mm pads as on 964t & 964RS w/ 1 piece 304x32mm rotors, RS, tt and street GT2 use the large 4 piston calipers(aka Big red) w/ 2 piece 322x32mm rotors.
964 steel uprights top, 993RS aluminum bottom
964RS steel uprights
993 n/a aluminum uprights
stock 964 front
stock 993 front
964RS/t front(the black one not the red one)
993RS/tt front
993 uses increased caster, increased track(by 25mm), incresed negative scrub radius(10mm), all give a "sportier" feel to the front end
Steering
Both 964 and 993 have power assisted steering, however 964RSA and RS manual racks are available
steering ratios
911 is ~3 turns lock to lock, non assisted
964 is ~2.81 turns lock to lock, assisted
993 & 993GT2EVO is ~2.47 turns lock to lock (2.73 for 993RS), assisted
964 manual ~2.6
the rubber "cushion"in the stock 993 tie rod is harder than in the 964 and the RS is harder yet.
ABS
both 964 and 993 base models use pretty much the same 3 channel ABS w/ rear pressure limiters(aka p/v). 993 M220 option adds 4 channel ABS w/ dual pressure limiters
Master cylinder & boost
964 n/a use a vacuum actuated 20.6mm m/c and 8" vacuum can w/ a boost ratio of 3, 964RS and 964t use electro-hydraulic boost systems RS w/ a 25.4mm m/c and a boost ratio of 3.6, 964 t w/a 23.8mm m/c and a boost ratio of 4.8
993n/a use a vacuum system w/ a 23.8mm m/c and 9" vac can and boost ratio of 3.15. 993RS and tt use the electro-hydraulic boost system w/ a 25.4 mm m/c and boost ratio of 3.6
Wheels and tires
964 front can use from 6x16 w/ to 8x18 wheels w/ 225/40 to 235/40 tires, though the latter get very tight, the lower the car the tighter it gets oem RS came w/ 7.5x17ET55 & 205/50x17 tires
993 front is very comfortable w/ 8x18 & 225/40 or 235/40 tires even at very low ride heights oem RS came w/ 8x8ET52 & 225/40x18 tires
964 rear can use from 7x16 to 10x18 w/ 265/35 tires though again the rear is very tight, largest factory fitment was 9.5x18 on n/b and 10x18 on w/b. oem RS came w/ 9x17ET55 & 255/40x17
993 rear has much more room 10x18 w/ 265/35 are a very comfortable fit even on very low cars