Someone once gave the best career advice to me and I still practice it till today. In life, having options is more powerful than any amount of money or pay. In another words, pick something and work towards something that makes you 'marketable'. You want to work towards a stage in your life where headhunters are calling you on a regular basis. In the old days, the gratuity package for army, police or even some petroleum companies is sizable. These days, with the current cost of living, it is very little. Anything less than 200k S$ by the time you retire at 50 or 55 (I am talking about 25 to 30 years down the road) is not enough if you work in inflation and consider time value of money. The gratuity pay out has not kept up with inflation and jobs in the army and police are not as attractive as it used to be. To give you an example, 15 years a go, I knew of someone cashing out 150k S$ and that was sufficient to pay for his son's overseas education with some left for retirement. Today, 150kS$ will barely pay for 3-4 years education at some Ivy league tuition fees.

If I were you, I will pick the chemical sales job. you are still young, easy to change jobs and career direction if it doesn't fit you. Once you join the police and get used to 3500/month, it will be difficult to adjust to a lower pay if you decide to leave the force.