hi Airwik,
Welcome to the forums! i have been watching your team develop this season and have been impressed with how you have gone, a real definite threat in the future!
If i were you i would keep them for now and try to pump up their Outside Defence and that would probably send their values sky-rocketing... to be honest, prolific OD is probably not high enough to attract the big $$ transfer offers, i think you would want them both at wondrous/marvelous to really get some value.. and with their potential, that is no problem..
Train pressure next season, get their OD up, and then if you still want to sell, then do it just before the playoffs next season to maximise income...
Whilst your training them, get your bleachers up to 10,000 to maximise more revenue from your arena.. with more income, you shouldnt have to sell those 2 monsters to get the support players needed to succeed!
Also, are either of those guys any chance for their U21 teams? if so, that should bring you some extra income in merchandising...
As far as i am concerned, those 2 blokes are the future or your team, and if mine, i would never sell them!