Opinion.... sell my rental and just invest in the market for the next 20 years

I own a rental in ontario that I owe $400,000 on at %4.2. After everything it cash flows approx $300 a month.. problem is I took out a $120,000 mortgage against my primary residence 6 years ago to fund the down-payment on the rental as well as some upgrades at time of purchase. This costs me $750 in mortgage payments per month. I also wnd up paying approx 3,000 per year in income taxes because of the rental. Does it make sense to just sell and invest the approx $8400 ((750-300) x 12 + 3000) per year in the market for the next 20 or just ride out and take the loss knowing that I'm building equity in the rental?

Thanks for reading