If you are looking to grow your portfolio and stay in the residential space I would definitely be an advocate for the HELOC. Origination fees are likely close to $0. Depending on the lender you can go up to 70% or 80% LTV. You don't have to use it all at once and you only pay interest on what you are using at that time. You could tap that equity for the down payment on way more than one more rental if you wanted to. You could also let each new property pay back the money you borrowed against this one to eventually pay the HELOC back down.
If you are looking to really go for it and get into commercial/ apartments I would sell this and go for the 1031 as suggested by @Caleb Heimsoth. If it's really worth $300k now that could be 25% down on a $1.2MM building or complex.
If you are looking for piece of mind than you probably want to stay put.
Personally I think leaving all the equity in that one house is a waste. Right now you are guaranteeing low risk and low returns. You don't need to leverage everything to the max and I wouldn't recommend taking on more risk than you are comfortable with. Some think it's less risky to have moor doors because you aren't hit as hard by vacancy, can take advantage of economies of scale, etc.