Smoke Detectors - 2-wire vs 4-wire pros/cons

What's are the pros/cons to using each, the 2-wire and 4-wire smoke detectors?
 
Can I power one of them off the omni panel?  Both?
 
Daisy chain?
 
Just trying to get a better understanding and the manual read confusing to me...
 
Suggestions on each?
 
I want to get them installed in the house soon and want to get them ordered/selected.
 
Thanks.

I'm deciding that now as well.  I am leaning toward the 4 wire just because 4 wire smoke alarms can also have heat detectors built in, and because 4 wire alarms seem a bit more standard, where only certain models of 2 wire smoke alarms as specified by the masnufacturer. The negative is that you do need an end-of-line relay to monitor power for 4 wire smoke alarms, but these are cheap and I don't think that is a big deal.
 
There weren't all that many to pick from. I just bought the Napco Firewolf detectors that claim better performance than previous detectors. (Or so they claim.) 
 
Note that monitored smoke alarms are almost always photoelectric type, because they have fewer false alarms, but can be slightly slower to react. Ionization type are used for the non-monitored smoke alarms because they are quick to react, but often have false alarms.  DON'T remove the other smokes when you add these. They are required by law.
 
Yes, 4 wire or 2 wire smoke alarms are powered by the panel.  They both are wired in a daisy chain.  The relay goes at the last one in the chain for 4 wire smoke alarms. 

The performance is a moot point. Put it this way, more OEM and others use System sensor devices. The point about heat detectors is a moot point, 2 or 4 wire support the same, the only variable is if tandem ring is needed.
 
Anyways, using something like the I4 detectors, the related module is programmed/wired like a 4 wire unit.

I went with System Sensor i4 COSMO-2W 2-wire detectors, which are combo smoke and carbon monoxide. As DEL was stating the COSMOD2W interface module wires to the panel like a 4-wire detector. The System Sensor 2-wire detectors have additional features like maintenance notification, freeze trouble notification, and the EZ Walk test mode. Plus you don't need an end of line relay.