in disc, you a compacting all the radar modules into one disc. It pretty much has 360 degree coverage, because the disc is ciruclar.
with the beam, only the ends of the beam do scan. Normally, you get 120 degree of scan on both side, so you don't get the full 360 degree coverage. I'm not sure about this, but maybe you can fit more modules into the beam, so it gives better performance than the conventional disc shaped radar?
As for plaaf AWACS, it's kind of interesting to see all the different types of AWACS coming out:
1. there will be at least 4 KJ-2000 soon. Most people speculate 20 as the final count of KJ-2000, so we will need to buy a few more IL-76 for that.
2. the skymaster Y-8 AEW are pretty weak and they are just limited to 6-8 of them. I don't know how useful they can be really.
3. the balance beam Y-8 can form the hi-lo combination of AWACS with KJ-2000. It's interesting, the stats of Erieye Hawkeye seem to really overwhelm those of the conventional AWACS like Phalcon or E-3 or A50I. If Y-8 can achieve anywhere close to that, it would be great.
4. the disc Y-8 AWACS is probably just built because IL-76 are too expensive to purchase in great numbers.