hypherthreading of CPU

It really depends on the number of machines that you are planning to run on the server and if you are going to dedicate cores to them. Hyperthreading will allow the ESXi host to see the physical cores as each having multiple logical cores. You probably won't see any difference at all in the performance. ESXi is still going to allocate the resources in almost the same way even if hyperthreading is enabled.

The only time you will probably run into issues is if you dedicate two logical cores on the same physical processor to two different machines. This would be the same as giving the one machine one physical core.

Here is a VMware write up about it: (for an older version but the concept holds true)

http://pubs.vmware.com/vsphere-4-esx-vcenter/index.jsp?topic=/com.vmware.vsphere.resourcemanagement.doc_40/managing_cpu_resources/c_hyperthreading_and_esx_esxi.html

you can also look at the 5.1 best practices guide
p. 20 discusses hyper threading

http://www.vmware.com/pdf/Perf_Best_Practices_vSphere5.1.pdf

There is no penalty.