This thread covers the career, which personally I don't play:
https://www.gtplanet.net/forum/threads/pc2-career-thoughts.393067/
Do you have a wheel? If so definitely get the game if not you should be fine if you are above average with a controller.
Buy the Deluxe Edition instead of the basic game as it contains very good DLC cars.
Advantages over GTS - logical car list with cars in proper classes particularly the historic race cars.
Multi-class racing.
Adjustable AI which isn't chase the rabbit, but can provide a proper race.
Dynamic time of day - e.g. night into day or day into night Le Mans style simulations.
Adjustable weather.
Adjustable field of view so you can tailor the cockpit view to suit etc.
Much larger track list including real historic circuits (I think 3 or 4 times as many circuits as GTS)
More realistic tyre model and vehicle physics - cars will be more realistic, but sometimes harder to drive and less forgiving.
Live track - rubber builds up, grip levels vary etc.
No grinding - all cars immediately available to use without wasting time grinding for credits.
Large number of different car classes so possible for players to race cars which they are interested in.
Large number of race car liveries.
Disadvantages over GTS
No livery editor
No Scapes feature
Some cars hard to control
Online less popular so best to join a league for online
No race numbers for road cars
If you are wanting a change from GTS also consider Assetto Corsa Ultimate Edition, but again probably requires that you be above average with a hand controller or have a wheel. Wreckfest - very entertaining - surprisingly good racing, tracks and wheel force feedback.