Lower cost is nice, but if a VoIP service doesn't have the features you need, it won't be a bargain.
Table 3 compares the features available with the two services. The Skype features include those in basic Skype, plus SkypeOut and SkypeIn. The Vonage features are the base set found in all of their plans.
|Analog telephone device provided||Yes||No. Optional|
|Incoming call rate centers covered,
(i.e. local number availability)
|>2000 in US||>1500 in US|
|Protocol(s) supported||SIP||Skype proprietary|
|Codec(s) supported||G.711, G.726, G.723||iLBC|
|Local Number Portability (LNP)||Yes||Yes|
|International Call Forwarding||No||No|
|Caller ID by number||Yes||Limited|
|Caller ID by name||Yes||Limited|
|Call-waiting caller ID||Yes||Limited|
|Disable call waiting||Yes||Yes|
|Three-way calling||Yes||Yes (4-way supported)|
|Softphone Support||Yes (extra charge)||Yes|
|Fax support||Yes (extra charge)||No|
|Retrieve from telephone handset||Yes||No|
|Retrieve via web interface||Yes||Yes|
|Receive via email||Yes via Voicemail Attachment feature||No|
|Modify basic/advanced features||Yes||Yes|
|Detailed call logs||Yes||Yes|
|Set default Codec used||Yes||No|
|Online address book||Yes||Yes|
|Click to call||Yes||Yes, from Contacts list|
|Web interface compatible with non IE browsers||Yes||Yes|
|911 Emergency Calling||Yes||No|
|411 Information||$0.99 per call||No|
|Free in-network calls||Yes||Yes|
|Technical support via telephone||Yes||No|
|Technical support via email||Yes||Yes, via online form|
|Account management by telephone||Yes||No|
|Table 3: Feature Comparison|
There are a few things worth mentioning that aren't included in the comparison chart, or that might not be readily apparent. The first is that there are currently no computer-free Skype options. So while options exist to use regular phones with Skype, all require USB connection to a computer running Skype. There are stand-alone Skype phones that have been announced by Motorola and BenQ, but they rely on a Wi-Fi connection and are smartphone / PDA-based.
Another consideration might be how each service handles emergency 911 calls. A recent ruling by the FCC (PDF link) mandated that VoIP service providers deliver all 911 calls to the customer's local emergency operator as a standard part of the service. While Vonage is clearly moving to comply, it's not clear whether Skype will head in that direction, at least not to the extent that "replacement service" VoIP providers must.
On a more positive note for Skype is that all of Skype's messaging, both text and voice, is encrypted with 256-bit AES (Advanced Encryption Standard). But VoIP encryption is a sticky subject, particularly in the U.S. due to its security concerns. So Vonage and most other commercial VoIP services are steering clear of it right now and run their networks without encryption.
Finally, note that Vonage lacks any of Skype's IM features, including the ability to see the status of someone you want to call and file transfer capability.