Multiple Engines???

It seems that for rockets of multiple stages with only one fuel combination,
there is an interesting engineering decision.

Consider a two stage rocket where both stages burn the same fuel
combination.

You could use 1 engine for the upper stage, and 4-ish engines of the same
design for the lower stage. The advantage is that you need only design one
engine type. The disadvantage is that with 4-ish engines on the lower stage
you probably cannot tolerate an engine failure, and clearly not a catastrophic
failure. Therefore you might lose a bit of reliability (which you might get back by spending the saved money on reliability).

Alternatively, you might use two different engine designs, a large
and a small. This reduces the total part count while increasing the
total unique part count. It probably increases cost and reliability.

Does anyone have any numbers that might help convince which is the
better path? For example, is motor design cost a large part of the
overall vehicle cost? Are most failures due to motor failures? Is a
single large motor likely to weigh less and/or have a higher ISP than
a few smaller (but still large) motors?

Basically, anyone have any good arguments for either choice?

-Thanks
-Talleyrand

P.S. Is it reasonably easy to tailor an engine to
atmosphere or vacuum operation with changes to the engine bell;
things like turbopump and cooling systems can remain the same?