Aero
Transmission
964 5 speed w/ available 40% & 20/100 lsd option
993 6 speed w/ available 22/40 or 25/65 or 40/65 lsd option
stock 964 gearing is arguably better for sporting use but can certainly stand improvement, US 993s suffer from very tall gearing
Here's a comparison of most of the factory gearing
Engine management
964 and '94, '95 US cars & all n/a RoW cars use Bosch 55pin OBD1 compliant DME & main harness, 993 version is v2.1 and adds 64kbyte of data memory, hot film mass air-flow sensor & a throttle potentiometer. A single oxygen sensor is used w/ this system
993tt and US '96 - '98 versions use an OBD2 compliant, 88 pin DME a& harness, v M5.2. 4 oxygen sensors are used w/ this system, 2 pre cat and 2 post cat.
Oil cooling
both have a single large ducted oil cooler w/ a multispeed fan in the right front, this cooler is adequate for most purposes but both can benefit from adding a second coler in the left front.
Rear suspension & brakes
all n/a 964 including RS use 299x24mm rear rotors, 964 t uses 299x28mm. All use 1 of 3 versions of the small body 4 piston axial mount calipers using the same ~98x44mm pad as the front small body(also the same as 930 f/r)
993 n/a use the same size 299x24mm rotor(w/ a slightly different o/s) and small body 4 piston caliper w/ 98x44mm pad, 993RS & tt use a 322x28mm rotor w/ an axial mount version of the medium/large 4 piston caliper using the ~132x48mm pad
rear suspension, part 2
one of the biggest differences between 864 and 993 is found here
964 rear uses a semi trailing arm rear suspension not unlike that used on older 911s, it uses a coil over shock rather than torsion bars. I has the advantage of simplicity
993 uses a sophisticated(and expensive) multi link design
Facts
