[dan sw fl]

It Mainly depends whether you live at 22' or 52' N Latitude or perhaps in-between.
I'm going to guess that you may be near 40' latitude.

Not knowing a location within 2,000 miles makes it Very Difficult
to relate to your climate - Predominant HEATing or cooling.

Efficiency could well be related to response to:
Did you have an annual maintenance contract for the last 16 years?

GENERIC BEST-CASE TERMS
15 SEER would likely provide upto 40% less in SUMMER Electric operating costs.
18 SEER may ......... provide upto 70% .........................................
,,, in comparison to an old XE-1000

4 & 5-ton systems are not going to achieve the precisely 18 SEER performance.
That is why ALL Mfg claims are stated as "upto".

Your AVERAGE electric bill of $250 does not reveal much about your summer time use.
Your Summer time electric usage could be in the range 25% to 75% of the AVERAGE bill.

If you are going to save $240 - $320 /year (30% to 40% of ~$800 annual cooling operating expense),
are you willing to 'wait for' a ~14-Year payback to make an equipment selection of a 4 SEER increase.