I'd do a Return on Equity comparison against other investments. As you found out, owning a rental isn't risk free -- you need to be compensated for the risk relative to what you could be earning in other investments.
Oh, and that $30k? That's a sunk cost now, should not affect what you do going forward.
I'm going to make a bunch of assumptions, change as needed. Equity: $150k. Property Taxes: $3k/year. Cash flow after taxes, mortgage, fees, insurance, etc: $350/month, $4500/year.
If you sell and owe 20% cap gains on $100k of equity then I assume this means you walk away with $80k + $50k (the equity you don't owe cap gains on) = $130k. [Edit: Probably a little less after transaction costs.]
You should be able to easily make, conservatively, 3.5% annually by investing this lump sum in something comparable such as a REIT fund or intermediate-term investment-grade bond fund. This would yield $4550/year. With these specific numbers it's basically a wash, though since you don't *really* want to be landlords I would sell and invest elsewhere. A fund never needs new paint or carpet or repairs :)
If there are other reasons you want to be in RE (e.g. for diversification) then this can factor into the decision, doesn't need to be a decision made strictly on the numbers. But ROE is a good starting point.
Again, this is all fictional and based on my many assumptions. Run your own numbers for this specific case. YMMV.