Is there a difference between the plasmids of E.coli XL1 Blue and DM700?

Question

Particularly were ampicillin resistance is concerned....

Answer 1

The XL1 Blue we used in my lab in grad school was not intrinsically ampicillin resistant, it was tet or something, and that was chromosomally derived and not plasmid based.

If, by some chance, you're doing this http://www.bch.msu.edu/bchug/web/bch472/472lm10.htm
then the answer is based on your transformation efficiency (# of transformants per ug of plasmid DNA). One strain of E. coli is more competent than the other.