In all likelihood, memset() will be inlined by your compiler (most compilers treat it as an ‘intrinsic’, which basically means it’s inlined, except maybe at the lowest optimizations or unless explicitly disabled).
For example, here are some release notes from GCC 4.3:
Code generation of block move
(memcpy) and block set (memset)
was rewritten. GCC can now pick the
best algorithm (loop, unrolled loop,
instruction with rep prefix or a
library call) based on the size of the
block being copied and the CPU being
optimized for. A new option
-minline-stringops-dynamicallyhas
been added. With this option string
operations of unknown size are
expanded such that small blocks are
copied by in-line code, while for
large blocks a library call is used.
This results in faster code than
-minline-all-stringopswhen the
library implementation is capable of
using cache hierarchy hints. The
heuristic choosing the particular
algorithm can be overwritten via
-mstringop-strategy. Newly also
memsetof values different from 0 is
inlined.
It might be possible for the compiler to do something similar with the alternative examples you gave, but I’d bet it’s less likely to.
And it’s grep-able and more immediately obvious at a glance what the intent is to boot (not that the loop is particularly difficult to grok either).