Since you have referred to NetScout, it sounds like passive monitoring of application flows is what you have in mind. Concord's offerings are more of the device-centric NMS sort. NI's Observer is much more like Network General's Sniffer product – it has long been in competition with Sniffer and the various Fluke products. It isn't clear what problem you want to address and what methodology best suits you. Observer is somewhat different than your other candidates but not necessarily the wrong choice.
As observed by Network Magazine regarding Observe v10 release:
"The highlight of this year's release is remote processing of expert data. With this capability, data doesn't have to be shipped back to the management console for processing; it's analyzed right at the probe. The result is a dramatic decrease in the amount of data transferred across what may be comparatively slow wide-area links. The company will also release a full duplex 10/100 probe, available August 9th.
"Network Instruments has also beefed up capabilities for applications such as convergence, specifically improving VLAN analysis. Its wireless analysis capabilities have also been improved, with over 50 new wireless expert conditions added to the mix. There are also a number of premade custom reports built right into the box."
As noted, reporting has been upgraded. So it certainly is a candidate but best compared to the current Sniffer offering.
With the large number of frame relay circuits, topology and best practices becomes an issue. For example, if you are working from a star, and/or know that the data centers are the key resource to be monitored, then passively monitoring at their edge can easily be the solution for you.