SIP Softphone

Pros:

• Cost.  In some environments a USB headset might be all that is required.  The cost of the SIP Softphone software is small (~$50) or included with desktop call control software.
• Easy to deploy with almost no configuration required.
• Zero footprint. Takes up no desk space.
• Works with Software VPN Client. This makes deployment to home users easier if you can work out the quality/reliability issues.
• Intuitive expandable interface with long lifecycle. Using a full monitor, keyboard, and mouse the feature set can be expanded through software.  In the same way, the feature set can be limited for security or integration purposes. 
• In some environments a USB headset might be all that is required.  The cost of the SIP Softphone software is small (~$50) or included with desktop call control software.

Cons:

• There can be hidden costs – upgrading PC, upgrading NIC to support QoS, upgrading headset to change from RJ-9 to USB.
• Unreliable Audio Quality. The Softphone will be competing with other applications for resources that can adversely affect audio quality.  Additionally, limited bandwidth and QoS settings from the PCs NIC port can cause latency and jitter affecting audio quality.
• Limited Survivability.  If the PC is unavailable, the call cannot be controlled and is at high risk of being lost.  No way to dial out in an emergency situation or for help desk if the PC is down.
• Users may prefer hard buttons for some functions.  Many users still prefer hardware buttons for frequently used actions.  For example, mute, pickup/disconnect, volume control.  Many USB headsets provide this hardware interface, but it may increase cost.
• No handset or speaker phone. Some softphone clients do offer a speaker phone function that will change audio output device to PC speakers (if you have them).  Unless you have a laptop, chances are you will not have a second mic input.  You can find a USB handset to use with a Softphone; however, this will increase cost.
• Platform Support.  Support of a Softphone on multiple operation systems can vary.
• The Softphone will be competing with other applications for resources that can adversely affect audio quality.  Additionally, limited bandwidth and QoS settings from the PCs NIC port can cause latency and jitter affecting audio quality.

SIP Hard Phone (or IP Phone)

Pros:

• Reliable Audio Quality.  Hard phones are dedicated to provide quality reliable audio and will not be impacted by PC network and CPU performance.  In addition, many hard phone vendors are touting HD Voice quality.  Softphones can leverage HD wide band codecs but it will increase the CPU requirements.
• No hidden costs.  There are typically zero hidden costs to deploying a hard phone. 
• Survivabile. If PC is unavailable, a call will stay connected and new calls can be made.
• Handset and speakerphone included.

Cons:

• Cost. The cost for a basic hard phone is around $150.  Vendors are adding more and more functionality to these phones and increase the cost.
• Hardware has a shorter lifecycle than software.  No software maintenance fees, but investment may not last as long and there will be RMAs.
• Too many buttons can cause problems – cumbersome interface, training issues, integration issues.  If a hard phone is used in conjunction with a call control client, there might be confusion about which interface to use and when.
• Large Footprint.
• Managing hard phones can be a challenge.  Firmware and configuration updates without the use of a provisioning server can require many man hours.
• Deploying to Home Users can be a challenge. Unable to leverage software VPN.  Must use a SIP aware firewall, STUN or another uncommon deployment technique to support home users.  Hardware VPNs can be used, but this will increase the cost for deploying a home user.