lets start here.
Titanium rods, two main types, forged or machined from Billet.
Forged as Forged is better for almost everything, as far as performance and weight. It almost always cost more.
There are many manufactures as well, so design as well as how that are made, what bolts are used.
We have used Jet Forged rods, as well as Pankl, and machined rods from a number of makers.
With the pankl rods the design questions start with:
Bore and stroke
CC per cylinder
Red line
Over rev RPM (RPM over redline)
And finally how long do you want them to last?
This was interesting, we used to go with 30 hours, never had one fail.
They made rods for F1 when they were using different motors for qualifying, these were designed to last 15 minutes.
The shorter the life span the lighter the rod.
They used to make the Rods for all the F1 car teams other than McLaren, who made their own. I do not know if that is still the case
OEM rods in Ferrari's Street cars.
Ti rods
advantages light weight
Disadvantages, they stretch more than steel, so piston to valve clearances need to be increased, expensive!!, you must use special care with rod bolts as titanium galls to everything
Steel, many many types, some can be designed to be as light as Titanium, using the best steels and hardening processes, stiffer that Ti or alum, less stretch
Aluminum, very light, less expensive to make, stretches more that steel, a small scratch or mark can cause a rod failure