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What's the difference between:
fldname='MyTable1.field1'
replfldname='MyTable2.field2'
replace (fldname) with evaluate(replfldname)

versa

replace &fldname. with &replfldname.

What would be more efficient?
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Macro substitution allows any single VFP statement to be put into a string and interpreted at runtime. EVALUATE() functions only on expressesions which return a value -- a subset of what macro substitution can do. Name expansion is also a subset, but not overlapping with EVALUATE(), which allows a "name" (of a table, field, etc.) to be couched as a string variable rather than as a literal.

The less work required in parsing the string, the faster the operation will execute. EVALUATE() and name expansion have much less overhead -- because they have more limited capabilities -- and thus execute faster.

The one tricky case occurs in statements where macro substitution is performed once, but EVALUATE() is performed for each of a set of items. In this case, macro substitution will probably give better performance in specific situations and in general is the better choice.

An example of the tricky case:

lcSearchString = "ATable.cDesiredInfo = 'This one'"

SCAN FOR &lcSearchString && macro substitution performed once for the SCAN
ENDSCAN

SCAN FOR EVALUATE (lcSearchString) && EVALUATE() performed for each record of table
ENDSCAN

This is also something to keep in mind with SELECT statements where macro substitution occurs once for the entire statement.