"Would this make a big difference for 18 counts for loop?"

That depends on what you do inside the loop!
Obviously, if there is a lot of code inside the loop, the overhead of managing the loop counter will not be very significant;
On the other hand, if there is very little code within the loop, the overhead could be very significant!

That's why it can be quicker to simply repeat the code in-line, rather than use a loop (known as "Loop Unwinding") - but obviously at the cost of bigger code size.
As ever, it's a trade-off, and only you can determine the optimum compromise within the constraints & requirements of your particular application.

"I guess I have to try it and see the generated code."

Absolutely!

"The problem is I dont know 8051 assembly yet."

If you really need to be looking this closely at the timing of the code, then you really need to learn it!