to sell or not to sell

ok need advise.
here is the background info we own a unit that we have had for 4.5yrs, we have recently just purchase a townhouse that we now live in and the unit we will rent out.
here is the advise we need.
we could keep the unit and it will pretty much pay for itself (by the rent),or we could do the easy option and sell the unit.
keeping would mean that we are investing in a nice little nest egg for the future (currently the unit is earning 10x per week what we would need to invest in growth). but we are worried that the growth wouldnt continue.
selling would mean that we can put some $$$ back into our new place ( so still investing in the future), pay off debts (other than mortgage), go on our dream overseas trip and have some left over for a rainy day. it also means (because we have paid off debts and put money on our new mortgage less) repayments so more $$$ in our hands each week

im just worried that selling seems like the easy option because it matches our current needs, it almost seems like we can have our cake and eat it too.
we are not planning on having kids for atleast 12months and my employer has good mat leave.
part of the reason we didnt want to sell was to have as a "wage" for when the kids come, but it also means we dont get to go os like we want to. we could afford to live on the mat leave repayments and rics wage.

sorry just worried as there is the old saying that if it seems to good to be true then its too good to be true and selling does seem like the easy "to good to be true" avenue.