Code:
100000 Dividing Comparisons Avg: 0.0464 s
100000 Subtracting Comparisons Avg: 0.0457 s
---
100000 Dividing Comparisons Avg: 0.0449 s
100000 Subtracting Comparisons Avg: 0.0463 s
---
100000 Dividing Comparisons Avg: 0.0527 s
100000 Subtracting Comparisons Avg: 0.046 s
---
100000 Dividing Comparisons Avg: 0.0525 s
100000 Subtracting Comparisons Avg: 0.0455 s
---
100000 Dividing Comparisons Avg: 0.0456 s
100000 Subtracting Comparisons Avg: 0.0465 s
---
100000 Dividing Comparisons Avg: 0.0454 s
100000 Subtracting Comparisons Avg: 0.0452 s
---
100000 Dividing Comparisons Avg: 0.0464 s
100000 Subtracting Comparisons Avg: 0.0457 s
---
100000 Dividing Comparisons Avg: 0.045 s
100000 Subtracting Comparisons Avg: 0.0467 s
---
100000 Dividing Comparisons Avg: 0.0462 s
100000 Subtracting Comparisons Avg: 0.0464 s
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100000 Dividing Comparisons Avg: 0.0451 s
100000 Subtracting Comparisons Avg: 0.046 s
Grand Totals:

PHP Code:
1 Million Dividing Comparisons Avg: 0.04702 s
1 Million Subtracting Comparisons Avg: 0.04618 s 
Looks like dividing is actually faster, but, probably negligible.