Put it simply: the faster page is generated - the better for SEO and more pleasant to the user. I am not going to rant, I think all the conclusions you will make by yourself on the basis of the test results.
Testing server: 8 x Intel(R) Xeon(R) CPU E5-2630 @ 2.30GHz, 2GB RAM, nginx/1.4.7 + php-fpm 5.4.26, MySQL Percona 5.6.16-64.1. Channel- 1Gbit/sec. Magento 220.127.116.11 CE with demo-date installed. Testing - through Siege.
Magento store speed test configurations:
- Cache is on+js/css+ full page cache
- Cache is on+php-apc+js/css+full page cache+redis
- Cache is on+php-apc+js/css+full page cache+varnish
- Cache is on+php-apc+js/css+ full page cache
- Cache is on+php-apc+js/css+ compilation
- Cache is on+php-apc+js/css
- Cache is on+php-apc
- Cache is on
- Cache is off
Each Magento store speed test was conducted for 5 times (except for test with varnish, on 5th time Data Center decided that it was a DDoS attack.Chart 1 Average load time for the server (Less - better)
Chart 2 all request in 15 seсonds (More - better)
Undeniable leader here is - Varnish. But it doesn't mean that this is the best option for your store, weak sides of the varnish are: issuance of the old cache, blocking of the headers, memory leaks, performance decreasing, incorrect caching of the mobile themes.
Chart 3 Page generation time in seconds (Less - better)
Chart 4. Average generated traffic per 1 second (More - better)
On the assumption of information from the charts that represented above graphic value is predictable.
Chart 5 Average load time Inquiry per second (Less - better) I would like to pay attention to the configuration with Redis. It shows little better results than average cache. But daemon of Redis- server has used only one core of the server. This means, that with correct configurations it will use 8 cores of the system and expected hit rate would be around 550 inquiries per 1 second, which is about 40% more comparing with full page cache usage. Compilation method also could be a good solution it increases speed on 20% in average.To be honest, drop in performance with php-apc together with the full page cache became a surprise for me. As I have promised right to conclusions is yours. However as we see correctly configured Varnish+ FCP (full page cache) perform the best speed. The only problem is that you need strong knowledge and a lot of time to configure this services correctly.