Hm, although I'm late, maybe I can still help with mine strategy;
I keep fast trainers only if they have 5*. I'd rather have 5* player with slow growth (G) than 3* fast trainer who has same value. I don't think they are worth spending packs and time to reach 5* and then to train them even more. I usually try to combine (very) cheap 5* players to save money and (very) expensive if I have choice to choose, of course. So for example at level 12 I would buy slow G players listed for 20-25M to save money for few fast G players which are worth over 40M. On low levels like 1st one differences are even greater (300-500K for slow G, fast G over 1.5M) while on high levels difference is really hard to spot. On level 23 slow G players are worth 106-107M while fast G players are worth 108M. At the end all that matters only if player is in form. I used to keep weak players which were in great form and sold high Q players which were not in form. It is really painful when you have to sell scout or better player because his form is bad. Before selling such player try to bench him or to place him on different position, that could help sometimes.