Personally speaking, I'd say that if you have someone that good now, then hold on to them, as the value is going up quicker than you could possibly earn in interest. The odds of being able to buy someone like Hook again are small I'd say.
The core of my team will stay with me for the rest of their careers, (Or at least that's the plan ATM)
Grrening, 22YO, Wond/Strong Bat
Staveley, 23YO, Rem/Sup Bat
Noon, 22YO, Sup/Rem Bat, Sup WK
Akhtar, 17YO, Resp/Med Bat
Bewers, 22YO, Rem/Sup LM
Wilde, 19YO, Sup/Resp RM
The rest will be replaced over time as and when I can afford to.