I always keep 3 ST in my squad. I play 4-5-1V (one ST) and 3N-1-4-2 (two ST) regularly and like to sub one almost every game, so it makes sense to always have 3 in case of injury.

Sell the one with the lowest value. Stats mean jack squat. The ST with the higher worth will train faster and be worth more to you and to others when you eventually sell him.