We just decided on the Cisco as our new system. We have about 2500 users and about 20 sites, including Europe and South America. If you want VoIP, this is the way to go based purely on the technology. Cisco is the only Enterprise class system that is VoIP from the ground up. If you are not dead set on IP then the Lucent is a good system.
Lucent, Nortel, NEC, and any other system that offers VoIP is doing it on there existing TDM platforms. Therefore, all of the system processing is TDM not IP. You just get the ability to use your existing data network for phones.
Also, to add IP phones or IP trunks you need to buy more IP cards. With the Cisco system, you use your exiting routers switches, etc. You are installing servers and gateway devices. The gateway devices are just like routers, but with voice enabled IOS.
The place that Cisco really shines is at the remote site. To operate a remote site, all you need is a 2600 series router with voice enabled IOS. If you use the IOS Plus, those phones can make internal and external calls through the router if the line to the central Call Manager goes down. If you normally would install a small PBX at that site, you just cut your install time from weeks to days, based on availability of hardware.
The only downfall to the Cisco system is the lack of features compared to a traditional PBX. Out of the box it does not do everything a traditional PBX will. However, once you add ACD, IVR, etc. You can get it to where you want it.
Long story short, if you want IP, I would recommend Cisco. We did a 2 month demo and have been researching it for about a year. The advantage to the Lucents, and Nortels of the world is that you can still use digital phones along with IP. However, it is not true IP. It is IP on top of TDM.
Now don't hold me to that, because I know nothing is perfect. However, we just went through the same thing you went to and everything I just mentioned is the thought process we went through to reach our decision.
One final thing that really hit us was that we felt that IP was definitely the future. You would have to think that the traditional manufacturers will be coming out with all IP platforms in the future. If we buy a TDM based system now, will we have to buy an IP platform in the future. We decided to go ahead and make the jump now.
I have much more information than this, but I don't want to write a novel.