Are they successful rentals? Cash flowing after CapEx, and in an area where you see potential for further growth?
My wife and I faced that same tough sell/keep dilemma, and for our rental we decided that keeping it, while retaining access to equity was the best of both worlds. It's been a great decision thus far. We've locked in the access to equity, enjoyed monthly cashflow and enjoyed continued appreciation.
We ReFi'd into a very specialized 1st position Heloc that's tied to a zero balance sweep account. So all of our idle funds and regular checking deposits sit on the outstanding balance and save us a ton of interest cost. All the while it's growing that available line to pull from. When the market turns, we'll simply write a check from our line and start making cash offers. It's great flexibility, you only pay for that access to equity when you need it.
Might be something to consider if you like the long term prospects of one of your properties.