I'm not familiar with National Communications. I currently subscribe to Monitoring Times, and just signed up for Scanning USA a couple weeks ago. There is also Popular Communications.
Monitoring Times is very good, but only a portion of it is dedicated to the scanning hobby. It also covers shortwave, and other radio related interest. A good portion of it is consumed by the Shortwave Guide, 18 pages in the issue I'm looking at now. It has radio, and software reviews, info on satilite, military, federal government, utilities, etc. This is probably my favorite. The issue I'm looking at now is 92 pages!
Scanning USA is all about scanning, and related equipment, techniques, software, frequencies, etc. It seems to be comming back to life, and is a lot more interesting than it had been for awhile. A good friend of mine, Les Butler, writes articles for them too! I bought a few issues off the rack, and was quickly convinced to subscribe to it! The issue I'm looking at now is 66 pages.
Popular Communications is a mix of everything, including ham radio. Only a small portion is devoted to scanning. I've never been impressed enough to subscribe to it. Although I check it out at the stands when I see it, an usually end up buying a copy. The issue I'm looking at now is 80 pages. :?