You'll find that once you reach a particular level, you're going to move to apache2-mpm-worker + fcgid for php, at which point, you might as well just use Nginx as a single solution.
You could use a CDN, though, if you decide not to use one from the start, consider making it easy to split off a hostname or make it easy to specify a separate domain for static content in your system. While static.yourdomain.com is nice, any cookies you set with .domain.com will be transferred to static.yourdomain.com which is a tiny bit of extra traffic with each request for a static resource.
Nginx/fastcgi is going to be marginally quicker than apache2/mod_php and roughly equivalent to apache2/mpm-worker with fcgid. Simplify your stack and use only Nginx in this case, consider using caching where you can. Remember speed is important, serving high traffic volumes is sometimes more important. Sometimes serving 100 people very quickly is not as good as serving 10000 people fairly quickly. mod_php5 uses prefork which does not handle the thundering herd very well. If your traffic is bursty, you'll have to deploy with fcgid/fastcgi.
There are some corner cases where Nginx/fastcgi introduces some bugs that you won't see with apache2/mod_php5, but, those are mostly involved with the environment variables and the way php talks with Nginx. Remember that Nginx's proxy is http/1.0 - even though Nginx talks to the client with http/1.1 - keepalives between Nginx and your proxy aren't supported.
That being said, if you wanted to stick with Apache, mpm-worker + fcgid/php5 can still do very well and if you offload your image content to a CDN, you've got a situation that most people can easily diagnose. Nginx doesn't support mod_rewrite in the same manner, there are modules you might use in apache that you can't use with Nginx, .htaccess directives are not supported (though can be emulated in the Nginx config).
If you are used to developing in an apache environment, you might find Nginx to be confusing. If you are developing a project from the start, you might find it easier to deploy today with Nginx and when you hit those issues, you'll have already addressed them. If you deploy with Apache today, and later try to convert, you might run into some issues that you didn't anticipate.