Which tool is better for packet analysis Wireshark or OmniPeek ?

"Which tools is better" is the same as asking about "PC vs. Mac", "Windows vs. Linux", "iPhone vs. Android", etc. - not really a good question to ask, because it depends on what you need.

Both Wireshark and Omnipeek are good tools, both have their strength and weaknesses. The two things where nobody will ever be able to beat Wireshark are

1. Price (it's free)
2. Dissectors - the packet dissectors are too numerous to count and decode things most other tools have never thought about

Things were Wireshark can be less optimal to use for are

1. high speed packet capture (with "high speed" starting at about 300MBit/s) - other commercial tools come with specialized capture hardware, but to be fair you can use some of those cards with Wireshark, too
2. protocols that are proprietary and no documentation available. Some commercial analyzers like Omnipeek are able to decode some of them (with an NDA) e.g. Citrix ICA
3. fancy graphics - Wireshark is very technical and does not really produce eye candy (meaning: things that you can put in a report that the CEO has to understand, at least partially)
4. Advanced diagnostics, e.g. when comparing multi point captures

There are probably more things, but any network analyst worth her/his salt will tell you that they combine different tools to get their results. Usually, Wireshark is the most trusted tool when it comes to decodes.