On Balance, A 3PO-series Protocol Droid is more useful than an R2-series Astromech droid

The 3PO-series protocol units were considered the most advanced Human-cyborg relations droids in the market for over a hundred years.[1]

• R2 less desirable in conditions with antigravity, such as in many advanced spaceships, than R4 units.[2]
  
• R4-units are employed before 22BBY and has been already used by the Jedi Temple, as shown in Ep2, Obi-Wan and his starfighter. R2-units has probably already lost its monopoly on astromechanical functions.
• R3 more desirable in conditions where large memory storage is needed than R2[2]
  
• Among less wealthy customers, R5-units are more desirable as they are cheaper.[2]
  
• We can get a live example of this: Uncle Owen at the start of Ep4 initially chose the R5 unit and C-3PO to work at his moisture farm. Until the R5 unit broke down to help a brother out(R2-D2), Uncle Owen would have not even wanted the R2-unit.
  
• In the same example, C-3PO was seen speaking about his ability to understand and speak Bocce(a language on Tatooine, despite that he never intended to land on Tatooine in the first place if not for evading Darth Vader's inspections) as well as being able to operate binary load lifters, something that is expected for a skilled human to do. Essentially, C-3PO doubles as a big-brained human.
  

• Most people on Tatooine have no need to fly to other planets, at least not with an aircraft that requires R2-units.
  

• The R series competes with itself more than the 3PO series competes with other Cybot lines.
  
• The R2 is the least exclusive droid out of the R2 through R5 lines. Therefore, it is effectively reduced to being a normal space mechanic for any entity that can afford R3, R4 or R5.
  
• The 3PO has no such weaknesses for decades and are considered to be the most exclusive and the most able droid of the field.
  
• A large portion of the R-series' market does not effectively use its full potential, rendering it just a moving tool kit for people that has no demand of moving outside the home planet.
  
• 3PO droids can operate starships, operate moisture vaporators, and can use tools. This leaves a very small set of things, for the average droid, that the R2 can do while the 3PO cannot.
  
• 3PO droids can translate between millions of languages, which is almost always useful.
  

• Star Wars Episodes 2, 3, 4, 5
  

Astromech droids, also referred to as astro droids or mechs, were a type of repair droid that served as an automated mechanic on starships. These compact droids used tool-tipped appendages stored in recessed compartments. Many starfighters relied on astromech copilots to control flight and power distribution systems.[2] Sitting in an astromech socket exposed to space, the droid, in addition to its piloting duties, could calculate hyperspace jumps and perform simple repairs.

The R2-series astromech droid was a model of astromech droid produced by Industrial Automaton, boasting a level of success that was never equaled in Industrial Automaton's long history. A combination of excellent design, high-quality marketing, and good timing made this astromech droid one of the most sought after droids in history, and one of the few vintage astromech series still in active production decades after it was first designed, including the prototype R2-0. The line was so successful, even being given a positive review by Mechtech Illustrated for its versatility,[3] that the Galactic Empire would later take credit for the design. The average cost for an R2 unit was about 4245 Republic Credits.

Functions

Technical schematics of an R2 unit from one of IA's HoloNet advertisements.

When not compared to each other, the R2-series shell was near-indistinguishable from that of the P2-series astromech droid, prototype of the entire R-series. The main difference was in their height - at 0.96 meters, the R2-series was less than half as high than the hulking P2. Like its forerunners, the R2 astromech was designed to work in and around space vessels as a diagnostic and repair unit. But unlike the clunky R1-series or the overly large P2-series, this rounded, waist-high droid was made specifically to fit in military starfighter astromech slots. This was a radical departure, as previously all such droids had been dedicated government models. The droid was equally popular with Galactic Republic fighter jocks as it was with the general public.

When plugged into a T-65 X-wing starfighter, Eta-2 Actis-class interceptor, or similar starfighter, the R2 monitored flight performance, pinpointed and corrected technical problems, and performed power management, optimizing shipboard systems. Its ROM library had reference and repair designs for well over seven hundred starship models to ensure it was able to do its functions in repairwork, with its Intellex IV internal computer scans technical files also co pinpointing potential problems as well as finding data patterns or debug computer codes, allowing it to conduct over 10,000 MPF operations a second.[3] The unit could store up to ten sets of hyperspace vector coordinates in the RAM of its astrogation buffer (thus allowing for even ships with limited navicomputers to make hyperspace jumps and also acting as a backup in the event of navicomputer malfunctions),[3] and many had the intelligence and experience to perform engine startup and pre-flight taxiing. The R2 operated flawlessly in the vacuum of interstellar space. R2 units were equipped with a holographic recording mode.[4]

• monitored flight performance
  
• pinpointed and corrected technical problems
  
• performed power management, optimizing shipboard systems.
  
• Its ROM library had reference and repair designs for well over seven hundred starship models to ensure it was able to do its functions in repairwork
  
• Intellex IV internal computer scans technical files also co pinpointing potential problems as well as finding data patterns or debug computer codes, allowing it to conduct over 10,000 MPF operations a second.[3] 
  
• could store up to ten sets of hyperspace vector coordinates in the RAM of its astrogation buffer (thus allowing for even ships with limited navicomputers to make hyperspace jumps and also acting as a backup in the event of navicomputer malfunctions),[3]
  
• many had the intelligence and experience to perform engine startup and pre-flight taxiing. The R2 operated flawlessly in the vacuum of interstellar space. R2 units were equipped with a holographic recording mode.[4]