Whatever works for you. Taking a look at your transfer history, it's obvious that you do day trade and it's been somewhat successful. I disagree with your intimation that building your Arena is any less successful, as I've had success with it thusfar and have seen quite a few other teams use that strategy to great success. As you said, your day trading has worked for you as well. That's one of the nice thing about the game, there's no "one right way."
However, when you come in asking about a way to cut down on your salary costs you're obviously worried about your financials versus the return on them. Looking at that and the strategy for your team in that you're not going for hard training, it makes no sense to use anything over a Level 4 or *maybe* a cheap Level 5 to get the results you want. In the mean time and during the playoffs, again, it may be worth it simply to drop the trainer.