Dialogic needs no introduction, so the Dialogic D4 PCI board. Synway and Digium too have D4 PCI equivalent Voice Boards. While both Digium and Dialogic boards uses PCI slots, Synway 4 Port Board uses a USB port like modem.
I have been thinking about comparing these three boards using few common parameters.

1. Ease of Installation
2. Easy availability and price
3. Software API Support
4. Technical Support
5. Developer Base
6. MTBF ( Mean Time Between Failure).

1. Ease of Installation
My ranking would be 1. Synway 2. Dialogic 3. Digium. Synway uses USB connector, so one does not have to open the cabinet of PC to install it! This I find to be a major advantage over Dialogic and Digium. Though installing the cards inside the cabinet may make the set-up neat and clean, but ease of installation seems more important here.

2. Easy availability and price
On price, Synway scores much higher than both Dialogic and Digium. And all of them are now a days easily available in almost all countries. Dialogic may have a little edge over the other two, Synway and Digium here.

3. Software API support
My ranking would be : 1. Dialogic 2. Digium 3. Synway. Dialogic is definitely ahead of the other two when it comes to software API supports. Dialogic has vast resources as well as Dialogic-trained manpower all over the world. Dialogic has very well developed APIs which can be used in three major operating systems like Windows, Linux and MAC OS. While Digium has open source Asterisk on its side, Synway claims to support all Dialogic API.

4. Technical Support
My ranking will be again 1. Dialogic 2. Synway 3. Digium. I have not worked with Digium, so I am relying on information available on internet and information by my friends. I have first hand experience with both Dialogic and Synway. Dialogic has very good technical support team as well as vast resources. Dialogic has a very active support forum www.dialogic.com/den. Though both Synway and Digium has active forums, but I think Dialogic scores over them.

5. Developer Base
No credit for guessing it correct, it’s Dialogic! There are many Dialogic developers in the world! Also there are many third party visual development tools based on Dialogic boards! One may check this tools for developing IVR Software which I posted earlier. Digium should be placed at second and Synway at third. But developer base for both Synway and Digium are increasing. That includes me too .

6. MTBF ( Mean Time Between Failure) :
I have seen Dialogic mentioning MTBF in their data sheets which is more than 5 years! So, I am quite positive that Dialogic must come at top here. Overall, now a days all boards are equally good!

So, overall, Dialogic may have an edge over other two CTI boards! But in a price sensitive market like India, Synway might find more supporter than Dialogic.