litespeed is a +1 selling point. but, apache plain and simple.
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Apache is free but LiteSpeed is good in speed and its better for handling high traffic sites and it is completely Apache interchangeable so LiteSpeed Web Server can quickly replace a major bottleneck in your existing web delivery platform.
I suggest having two servers with one on each. I say this because some users might like apache more and others litespeed. In addition you can benchmark both and see what one is more suited for your needs.
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LiteSpeed is lot faster than apache, specially the enterprise version. Given the high traffic you get, I think you should make the switch. it totally worth it.