hmmm. I am not sure now. Which one buy for career single player only - 19 or 20
The 20 career mode is far more involved and adds all these elements as part of managing yourself and the team:
+ You appoint a manager who finds you realistic team interest based on your performance and reputation shown by the number of fans/followers it's very Twitterish in that respect. As the rider you earn signing bonus, a wage for each event and bonuses for good performance, this you can spend on hiring better staff.
+ Once you join a team you hire the Chief Mechanic and Data Analyst and they have skills across the 4 R&D areas of Engine, Chassis, Aerodynamics and Electronics (only Engine and Chasis development are available in Moto3). The Chief Engineer and Data Analyst also have a synergy rating for how well they work together so it isn't just about choosing the person with the best stats, it's also about the best teamwork as that boost Research Points which are awarded after each race event.
+ The development tree for each part (engine, chassis) is detailed into several subparts which all cost Research Points to build. It isn't as detailed as the F1 tech tree but it's way more detailed than 19 which was just a 3 section bar for engine and chassis. You assign your team technical staff to the projects (6 available in Moto 3) and this speeds up the research time - some parts take several weeks to develop. If you're not building any new parts you can assign them to gain more research parts for the area you want to focus on.
+ Events in 20 are properly spaced in the calendar so you have development weeks between events (19 just rolled to the next event)
+ During Free Practice you can take part in 3 bike development tests, fast laps (2 laps), race simulation (5 laps) and a currently seemingly broken 'follow the race line' 2 lap event. Your performance in each adds Research Points that you can spend in the tech tree (All very much 'borrowed from F1).
+ After each race event, you may have discovered new staff with better skills that you can hire. There seems to be no penalty for changing staff so it's just about finding the ones that fit your team and synergy rating best.
+ 20 has bike damage that will take your crew time to fix should you fall off (even if you use the rewind function, which is either interesting or a bug!)
+ 20 has the 3 section inner, middle, outer tyre wear which gives a much better indication of tyre wear by tyre type. The high-temperature tracks really show the difference in wear on the soft tyres too.
+ 20 has fuel management and power modes for the engine that allows you to vary the fuel use and slightly increase performance (plus you can develop parts that improve engine economy) so it's much more in-depth in the choices you can make. Remember to add a lap of fuel for the formation lap too (even if you don't run it the fuel use still shows), though you can, of course, lower the power to conserve some fuel.
- On the other hand I hear what people are saying about some AI balancing issues and although I haven't directly experienced them (other than some AI bikes in 20 being fast in corners and then slow on the straights) and I'm sure they are right - there will be kinks to be worked out in 20 through patches.
By contrast, the 19 career is more basic but there are things to like:
+ 19 Career starts with the choice of Redbull Riders for a short 7 race series before Moto3, I quite enjoyed it.
+ 19 is more about jumping from event to event with less management element - there are no Free Practice development mini-games.
++ 19 is an all round more polished game so the AI performance etc should be more consistent.
I prefer a more in-depth and connected management game so I see these elements of 19 as downsides:
- Bike development is simplified to 3 bars per team for engine and chassis only.
+/- The research includes developing TCS, AW and the ECU to make the bike more rideable, which is ok as a game mechanic.
- None of the staff management or synergy elements that 20 adds.
- No fuel management, single tyre wear indicator.
- No development mini-games.
I'm sure there are other people who would see the get on and race nature of 19 as an advantage so it depends what you are looking for. I like the depth and player control of the F1 games and similar WRC8 management elements. Both games are enjoyable to play, personally I prefer the 20 braking but again I see the comments from others that 19 had other advantages like more realistic lap times so there isn't much to split them in terms of gameplay.
If you want more immediate racing and less management go for 19.
If you want more team management and strategic choices in the races go for 20.