Kind of torn between the alternatives.
Found this quote interesting. Never thought of the opcache.
http://stackoverflow.com/questions/7147305/performance-of-variable-expansion-vs-sprintf-in-php
Ultimately the 1st is the fastest when considering the context of a single variable assignment which can be seen by looking at various benchmarks. Perhaps though, using the sprintf flavor of core PHP functions could allow for more extensible code and be better optimized for bytecode level caching mechanisms like opcache or apc. In other words, a particular sized application could use less code when utilizing the sprintf method. The less code you have to cache into RAM, the more RAM you have for other things or more scripts. However, this only matters if your scripts wouldn't properly fit into RAM using evaluation.
Sounds like single quote (nowdocs) catenation may be fastest when only a single var. But then there is that opcache thing and memory mentioned in the quote.