to be honest the reason you got so many iffy answers is because no one is certain about things like salary caps and potentials but none-the-less i'll try my best.
PLAYER A: just so you know training begins to slow down from 19 onwards and i think that even if a player were to star with a 6k salary, even trained properly he may never reach his full potential, there are so many factors to account for and its unlikely that a player won't suffer an injury in 5 seasons. Also as you may know training certain skills also depends on the level of other similar skills, i.e if you are training ID its best to have IS of a similar level.
And yes that may be true but with those kinds of players people normally sell them because noobs and people with a lot of cash just see the potential and immediately think the player will be epic. If you did have a player like that and wanted to train him you could by all means, especially if he is a center skills can make the salary jump very quickly and you could find that there is not much of a difference between similar players with one who had a higher starting salary.
NO NO NO!!!! Its a common misconception to think that potential has ANYTHING to do with training speed!!! Height, Age and skills i.e the higher the better and the more similar the better are the only things that affect training speed! Potential also doesn't affect how common pops will be, for example, if you are training 2 centres, 1 is 7 foot with all star potential and the other is 6'7" but with Hall of Fame potential there is no argument that the first will train faster!
It alllllll depends on what you are looking for, personally i'd still train a player with starter or star potential if i wanted him and if he suited my needs at the time. A lot of new players make the mistake of thinking that potential is everything and the higher the better, but truth be told you probably couldn't afford to have an HOF potential player and train him up, his salary would be too much for you to handle, especially if he is a center. Well i don't know your finances but they get expensive pretty quick. Further more you may only need a plyer who has a salary of around 20k to be top of your league and it may be smart to invest in 2 players with worse potential and train them up equally than to have a single player with epic potential getting all the training. At the moment you should be looking to develop your team and decide whether you are inside or outside orientated or training big men or guards.
It is true that in your case player B's training will begin to slow sooner provided their height and so on are the same. Just to answer your Q ;)
But you are new right? this is your first draft? You should be looking for 1 good player if not 2 and train them up equally in the hopes of developing a better team, not just a better player. Thats my opinion at least.
i tend to ramble but hopefully i helped i'm in a rush and have to go now but i can reply later if you want. Good luck :)