Long post alert! I have a LOT to say about this!
The first time I ever did a B-Spec race was the FR cup in GT4. I had an '05 Ford Mustang GT, the car I started the game with. I had a hard time adjusting to the new engine (My wheel was a piece of crap made in China), and the
understeer fact that it was a Mustang lost me quite a few races. I sat and stared at my tiny little screen as this faceless driver driving my car calculated every turn, took every apex with absolute presicion, and won the race when I (thought I) could not. I raced t again, this time in A-Spec, trying to remember what I watched him do and I won the race.
Fast forward a year and change, that Mustang is gathering dust and every main race is completed, as well as the first two licenses. I decide to B-Spec once more (I hadn't done it since that one race), so I buy another memory card and start another save game to use B-Spec ONLY. I watched him. I watched every race, and I never raced once. In that save, I never did a single license test, I didn't even do a single arcade race. When I finished the final race (The Infineon enduro, I fought with the tune for it for hours) I was extremely proud of myself for doing it. I had fought and fought and fought and spent countless hours tuning a Dodge Viper SRT10 '03 for the Supercar Festival, and
the day after I finished it I went to GameStop and bought a PS3 and a copy of GT5. I install the updates (18 hours of my life I won't get back) and sign on to find that they gave me a car to start with, the Toyota 86-GT. I spend the 20,000 on tuning parts, throw a mediocre tune on it and go to the Sunday Cup and lose.
The main reason I lost was because I used a wheel for GT3 and GT4, but was forced to use a DS for GT5.
I go to the B-Spec Sunday Cup, and I got a very warm feeling when my driver (Who's name was C. Beach) lapped the tailing car in a race I couldn't even win. I then made a resolution: I would finish every single B-Spec race GOLD before I started A-Spec. I had a tremendous amount of fun! I can't even say how many hours I sat on the edge of my seat, wondering if the non-tuned ISF Racing Concept was good enough to beat the menacing R8 LMS, or watching the last few minutes of the Nurburgring 24 (A race I will NEVER forget) as my driver's XJR-9 got in a huge crash in the last lap, and finished 16 seconds ahead of second place. To celebrate, I grabbed my X2011 Prototype and beat the Sunday Cup with it.
When I started the Clubman Cup, I had over 13,000,000 creds to my name, and a garage of over 280 cars. I end up using the Ford GT40 in the Classic Muscle Car race, and find that I had more fun in the B-Spec race where I used a GT350R.
TLDR:
I had more fun in the B-Spec events (In both GT4 and GT5)
than I had in the A-Spec events.